Time to 100% completion: ~12 hours
Minimum number of playthroughs needed: 2 but 3 recommended

I recommend the playthroughs in this order:
-first play on lowest difficulty (Seer), and collect all collectibles
-then do new game + on any difficulty, at the start of the game change difficulty to God and play through it (you unlock God difficutly by completing the game once)
-lastly do a speedrun (under 2h) by playing New Game + on Seer difficulty

These achievements do stack! So by playing on the highest difficulty you will also get the ones for playing on the lower difficulties!


For completing Chapter 1 on a certain difficulty:
(Chapter 1 ends as soon as you get the Jarngreipr from Henrik)

Rearmed Seer
Complete chapter 1 on Seer-difficulty


Rearmed Viking
Complete chapter 1 on Viking-difficulty


Rearmed Berserker
Complete chapter 1 on Berserker-difficulty


Rearmed God
Complete chapter 1 on God-difficulty


For completing Chapter 2 on a certain difficulty:
(Chapter 2 ends as soon as you go into the East Wing)

Gatekeeper Seer
Complete chapter 2 on Seer-difficulty


Gatekeeper Viking
Complete chapter 2 on Viking-difficulty


Gatekeeper Berserker
Complete chapter 2 on Berserker-difficulty


Gatekeeper God
Complete chapter 2 on God-difficulty


For completing Chapter 3 on a certain difficulty:
(Chapter 3 ends as soon as you collected the second skull key and came back to Midgard)

Traveling Seer
Complete chapter 3 on Seer-difficulty


Traveling Viking
Complete chapter 3 on Viking-difficulty


Traveling Berserker
Complete chapter 3 on Berserker-difficulty


Traveling God
Complete chapter 3 on God-difficulty


For completing Chapter 4 on a certain difficulty:
(Chapter 4 ends as soon as Loki kills you)

Mortal Seer
Complete chapter 4 on Seer-difficulty


Mortal Viking
Complete chapter 4 on Viking-difficulty


Mortal Berserker
Complete chapter 4 on Berserker-difficulty


Mortal God
Complete chapter 4 on God-difficulty


For completing Chapter 5 on a certain difficulty:
(Chapter 5 ends as soon as you absorb Hel)

The seer becomes a God
Complete chapter 5 on Seer-difficulty


The viking becomes a God
Complete chapter 5 on Viking-difficulty


The Berserker becomes a God
Complete chapter 5 on Berserker-difficulty


The God of Gods
Complete chapter 5 on God-difficulty


For completing Chapter 6 on a certain difficulty:
(Chapter 6 ends as soon as you kill the end boss (or maybe a bit later))

Seer of Ragnarok
Complete the last chapter on Seer-difficulty


Viking of Ragnarok
Complete the last chapter on Viking-difficulty


Berserker of Ragnarok
Complete the last chapter on Berserker-difficulty


God of Ragnarok
Complete the last chapter on God-difficulty



Tips for God difficutly
The method I used to play through the game on God difficulty is probably the easiest one, but it is also not intended to play it that way. The Devs said they wanted to fix that, but until now they didn't. Keep that in mind. Do the same or play through it regularly, I don't mind.

So the method is: play your first run on either difficutly, but collect all 15 Eitrium cells (the little ones). Then after your playthrough is done, go for New Game + and choose any difficulty. When you are ingame, as soon as you can, pause the game, go to options and change the difficulty to God.
Now you play on God difficulty in New Game + while having all the nice upgrades for your Jarngreipr as soon as you get it in chapter 2. The God difficulty achievements will still work. The rest should be mostly easy.
I would also recommend to collect most or all big Eitrium power cells, they can be very useful, especially for the boss fight, although you don't have to use them.
Also I would recommend to kill most of the enemies (if not all) while you can recharge your power cells at a machine.

What is God difficulty
-you usually die from a single hit, but then go in the ether as normally, though not everything insta kills you, for example the boss does not, or by getting fall damage
-even by saving you go to the ether, so I wouldn't recommend to save at every save stone!
Just save here and there, or if your last save is already quite old. But keep in mind if you fail the ether part while saving, you die and then have to continue from the previous save. So it is always a risk.
(Except as soon as you are in chapter 5 you can't go into the ether anymore, so saving has no downside anymore, so at this point save whenever you can!)
-more enemies

Warning concerning a bug
You should definitly save on God difficulty either before going into the giant worm or shortly after (both in Chapter 4). Because at two points inside this giant worm there will be some worms hanging from the ceiling. If you don't crouch under them, they will kill you. Now the bug is that your game will then freeze after they killed you which means you won't go into the ether, you will have to brute force close the game and restart from previous save.

Boss Fight
The only really hard part is the boss fight. On God difficulty you have to blast his face 5 times instead of 3 and he will summon way more worms. If you die here, don't worry, you have an automatic checkpoint at the start of the boss fight, so you can instantly try again.
My general strategy for this one is as follows: stay mostly on the sides of the boss to be able to shoot the eyes on one side. Then go to the other side. After every time you blast his face he will summon a bunch of worms coming from his direction, so simply wait a bit for them and do a single blast in front of them to kill them all, then repeat shooting the boss.
If you can, recharge energy from the ghosts, but this boss is also the last opportunity to use your big Eitrium cells, do that!

There are three types of collectibles in the game:
-the "collectables" (green floating stones) which I will call orbs, they are just for an achievement, there are 28 of those; you can check how many you have by pausing the game and looking on the right lower corner of your screen
-audio logs which give some story and an achievement as well, there are 28 of those;
you can check how many you have by pressing TAB and going to the audio log page
-the small Eitrium cells (the ones which you put in the Jarngreipr),
which are very important for the game and give an achievement as well, there are 15 of those;
you can check how many you have by pressing TAB and going to the homescreen

Then there are also the big Eitrium power cells which can be used once to restore one bar of energy, so they are not classic collectibles, but I will still show you the ones I found. However you do not need to find all of them. Just for the The one who saves... achievement you need to have 30 of them in your inventory at once. But as I found 71 of those and you never have to use them, that shouldn't be hard. Just try to save those. They are still useful, so why not grab them all? :)

In addition to the stuff above I will also add here all the codes in the game and the locations for all RFID keys as well as the location of all save stones and all energy machines I found. (Even though you don't have to use every save stone or every energy machine, but they help.)
I will not mention the enemy encounters though, except when enemies are directly in an area where a collectible is!

Also keep in mind if you missed some collectibles in your first run, don't worry to much, you keep everyything you found in New Game + (Except the big Eitrium power cells and the RFID keys), so you just have to collect the remaining stuff. At least I think so, I got all collectible achievements in my first run, so I can't confirm that. There is also a chapter select but I haven't used that at all.
Also The one who saves... achievement is way easier in New Game +, but simple enough in a regular new game.
And in general just play on Seer difficulty when going for all collectibles to make it easier.

I will mainly show you the locations of the stuff above in the order you can find them. Although in the "Apsulov Research" area you could also look for them in a different order, even in another chapter for some of them. But keep in mind all the stuff you can reach from the "Apsulov Research" area you have to get before going back to the West Wing in chapter 4 as this marks the point of no return!

Ross would be proud
Find all collectables


Historian
Find all audio logs


I’ve got the power
Collect all upgrades for Jarngreipr


The one who saves...
Have 30 power cells in your inventory

In the tutorial: code 1436 (you don't always have to use the code)

Right after the tutorial, you have to jump into the vent. You will get to the 1. audio log.

Shortly after you come to a door: code 6725

A bit later you will come to your 1. save stone. This is the only one which you have to use once!

After getting out of the first elevator, you have to climb over some boxes, on one of those is the 1. orb.

Shortly after you will drop out of the vent and find the 2. save stone.

Just a few steps forward, you will find the 2. audio log on the floor.

Right after the dog scene in the vent, you will have to turn right to leave the vent. Don't that now. Instead continue in the vent to find the 2. orb.

In the room where you see the dog again behind a door, you find the 3. save stone.

As well as another door: code 6725

Shortly after you need a code for the elevator: code 7913

After coming out of another vent you come into this room with the 3. audio log on a table.

The 4. save stone in the next room.

Shortly after turn right, over the stairs into an optional room with the 3. orb in one of the lockers on the left. (There is also an enemy patrolling in the area, but not on Seer difficulty.)

You will have to come into this room with the 4. audio log right next to the other door on a table.

Shortly after you will have to take this hand to progess.

After opening another door with the hand, the 5. audio log will be on your left on a small table.

Right at the start, after stepping up from the chair, you see the 5. save stone.

In the same room you will have to scan the hand to get RFID key 1. (It is mandatory to get.)

A bit later you will have to collect the 1. small Eitrium cell in a box. (It is mandatory to get.)

Next to the vent you have to shoot is the 1. energy machine.

After another vent is the 2. energy machine.

After the ladder, turn right, above the stairs and hack the door. (Have to activate your vision and then blast at the right moment, if you fail, go back to the last energy machine.) Follow it till the end to find the 4. orb on the left on a crate.

Now go back and after the stairs look on the table. There is the 2. small Eitrium cell, as well as the first 5 big Eitrium cells. (After picking up the one in the machine, another one will drop out once.)

Next to the next vent you have to shoot will be the 6. big Eitrium cell on a table.

Next to another vent you have to shoot will be the 7. and 8. big Eitrium cell on a table.

And that same vent, turn left immediatly to find the 5. orb at the end.

Directly after taking your first tunnel into Niflheim, turn around to find the 6. save stone and the 6. audio log.

Much later, in the little outpost is the 7. save stone.

Go through the door on your left and find the 7. audio log, the 9. big Eitrium cell and the 3. small Eitrium cell, in a box, all on the shelf.

Turn around and find the 3. energy machine.

After dropping down from the roof, find the 10. and 11. big Eitrium cell and the 4. small Eitrium cell in some lockers on the right.

Now check the left lockers and find RFID key 2 after finding the skull at the same place. (It is mandatory to get.)

In the same room, hack the door straight ahead to find the 6. orb next to the toilet.

When you are outside again, go straight into the next room (instead of going left) to find the 12. big Eitrium cell.

Back in Midgard, find the 8. save stone after the vent, to the left of the door.

When you see the dog, turn right (where you comes from) and follow the way into a room with the 8. audio log in the last locker on the left.

Continie your way, and soon after you will find the 9. audio log directly infront of you.

Enter the Apsulov Reserach area and infront of you, to the left of the stone plate is the 10. audio log.

Now the Apsulov Research area. This one is big and kinda open with a lot of doors to open with multiple floors, you need more keys for most of them unfortunetly. In fact you will need the keys 1,2,3,4,5 and 7. Only in chapter 5 you will have all the these, so we will skip these rooms for now, even though you could go into some of them already.

Go into the elevator and choose "4F: Surface Access". You will find the 11. audio log, as well as 4 big Eitrium cells, so we are 16.

Now go to "B1: Waste Disposal". Go straight a bit and find the 9. save stone.

From there, turn right and then right again. Go through the next door and find the 12. audio log, as well as the 17. and 18. big Eitrium cell on a table.

In the same room you will find the 7. orb in one of the lockers.

Continue with the story and you will have to open this door with the code: 3425

Later on you come into this room with a bunch of lockers on the left. In those you find the 8. orb and the 13. audio log.

After the airlock, find the next 5 big Eitrium cells, so we are at 23.

A bit later on, when you see this door turn left to find the 4. energy machine.

Now hack the door and find the 5. and 6. small Eitrium cell on the table, one of them in a box.

In this room hack the first door on your right to find the 24. and 25. big Eitrium cell.

Go out of the room and hack the next door on your right to find the 26., 27. and 28. big Eitrium cell.

Go out again, hack the next room on your right, go up the stairs and find the 29. and 30. big Eitrium cell on the table.

Later on, you will have to travsere over a pipe. Before doing so, collect the 31. and 32. big Eitrium cell, one on each side on top of some boxes.

When you see another of those big ghosts, turn left into the container and find the 5. energy machine.

Later on you come into this room witht the 6. and 7. energy machine, as well as the 10. save stone.

There is also a locked door here where you will need the code. if you already know it, you use it right away and skip a section of the game, but you will also skip collectibles, so don't it if you wanna go for collectibles. Otherwise check the speedrun section for the code or just skip a few screenshots here.
So, let's contiune and climb down one of the ladders.

Here are the 8. and 9. energy machine. (Yeah, a bit overkill here.)

After opening all the doors in this area, go left (in A2), to find the 9. orb.

Continue on till you find the code, then go back to the door: code 4673

After the airlock, right in front of you are the 7. small Eitrium cell, the 33. big Eitrium cell and the 14. audio log.

Right next to that is the 34. big Eitrium cell on the floor.

On the left here, is a door you can power. Do that to find the 11. save stone behind it. (Take back your energy after saving.)

From the power station, look on the right (behind the pillar) to find the 10. orb.

Go into the next room and the 35. big Eitrium cell in front of you.

Same room. Check the first locker on the right and find the 36. big Eitrium cell.

As well as the 11. orb in one of the lockers on the left.

Right before the next vent you have to shoot is the 10. energy machine.

Drop down from the vent, go down the stairs, take the other stairs up, turn right and scan the last body to get RFID key 3. (It is mandatory to get.) (There are enemies in this area patrolling.)

After inserting the skull key in the Apsulov tablet, go down the stairs next to it, go right and open the door to find the 12. save stone. (You can open this door earlier but then have to hack it. With RFID key 3, you don't need to hack it anymore. And yes, I said we would open all the doors in the Apsulov Research later, but I wanted to get this one now for saving.)

Now go into the East Wing: code 3825.

As soon as you are in the East Wing (start of the chapter), you can see the 13. save stone in front of you.

You will soon come to the 11. energy machine.

In the room where you have to unlock the door, is the 37. big Eitrium cell.

From the same room, hack the other door, go through till the end to find the 12. orb.

Go right, when you see some lockers into a bathroom to find the 38. big Eitrium cell.

You will come to the 12. energy machine on the left.

If you continue on the left, you will come to a locked door. We will need the RFID key 4 for that one.

Continue right instead into the airlock to find the 8. small Eitrium cell in a box.

From the airlock, turn right and continue into this big area. Continue your way on the left, don't climb down the ladders yet. At the end will be the 13. orb. (There are enemies in this whole area.)

From there, turn around, go down the nearest ladder. Turn around an go through the door on your left. 4 doors here.

Behind the second door on the right is the 39. big Eitrium cell.

Behind the second door on your left is an enemy with the RFID key 4. Blast him, then collect the key. (It is mandatory to get.)

Go back from the area with the 4 doors, go straight ahead through another door. Another 4 doors here.

Hack the first door on the right to find the 40. and 41. big Eitirum cell.

Hack the first door on the left to find the 42. and 43. big Eitirum cell.

Now go back to the level 4 door. Now you can open it. Go through it and through another door. In this bathroom on the right will be the 14. orb.

After going through the mandatory level 4 door, you find the 15. audio log on a table.

Very shortly after is the 14. save stone on the right.

Shortly after you come into an erea with a total of six doors. (They are enemies patrolling here.)

In the first room on the right are the 44., 45. and 46. big Eitrium cell. The rest of the rooms are empty.

Go straight here into the red corridor to find the 15. orb at the end.

Go back and turn right to find the 13. energy machine.

Go up the first pipe on the right, then turn right to find the 14. energy machine.

Right after the elevator find the 15. save stone.

Shortly after the 14. energy machine. (Enemies in this area.)

Same area. Go into the room on the right to find the 16. orb on the table.

Continue into the vent and find the 16. audio log.

Almost at the start of the ice area is the 17. audio log on the ground.

In the next outpost, is the 15. energy machine in the first room, on the right.

As well as the 9. small Eitrium cell on the table.

From there, take the door on the left, go left again and find the 10. small Eitrium cell and the 18. audio log.

Go straight from there into the next room, hack the next door and find the 47. and 48. big Eitrium cell.

Go back, up the stairs, turn left, hack the door to find the 19. audio log on the floor on the right.

As well as the 17. orb on the left.

After the airlock is the 16. save stone.

As soon as you go out of the cave, turn left and follow the left wall to find the 18. orb on the ground.

A bit later the 17. save stone in your way.

After the dog scene is the 18. save stone on the ground.

Right at the start is the 20. audio log on the right.

We are at the Apsulov Reserach again. Go back to the other side, put in the skull key into the Apsulov tablet and take the elevator back to "B1: Waste Disposal". Go back to Henrik. After some stuff here, listen to Henrik talking and then scan his hand to get RFID key 5. (It is mandatory to get.)

Back to the elevator. Go to 2F. Somewhere here will be an enemy patrolling. Blast him once and collect the RFID key 7. Now we have all keys we need for the Apsulov Research.

Back to the elevator. Now go to 3F. On the left will be a door you have to power. In there is the 11. small Eitrium cell.

Out of the room. Don't take your energy back yet! Go into the next room and find 5 big Eitrium cells, we are at 53.

Go out of the room and take your energy back. Then continue after the door to find the 21. audio log on the left.

Back to 2F. Now we pretty much go into every room in 2F, counter clockwise. (Take care of the enemy patrolling around, if you didn't kill his ghost.)

First door, is locked, go through second door, behind it, shoot the vent, go through and find the 19. orb.

Open the door in this room via the panel, go out and go in the next room. The 22. audio log is there on a table.

Next room. Find the 12. small Eitrium cell on the right.

From there, turn right at the next opportunity. Go down the stairs and find the 20. orb.

Go back and go in the next room. Code for the door: 2625. The 21. orb is in there on the right.

Next room has the 23. audio log on a table.

Next room has the 22. orb under the table at the back.

Leave the room through the other door. You are back at the Apsulov tablet now and done with 2F. Now go down the stairs and now in 1F, again, go in every room counter clockwise.

Skip the first room as we already were there. (There is a save stone inside.)
The next room you can enter is empty.
Next room you have to hack. It contains the 24. audio log. (As well as the code we used earlier.)

Hack the other door, go out and go into the next room. The 25. audio log is there.

Now we are done with this area and you should get Explorer Achievement. Go into the elevator and choose "B2: Waste Treatment".
In Jotunheim you find the 19. save stone directly in your way.

Directly after that, BEFORE stepping on the plattform. Look on the table to find the 13. small Eitrium cell, as well as the 23. orb behind the table. (You could also get those collectibles later, but better take them now.)

Now step onto the plattform. You are now in the worm!?
Do some steps straight forward to see the 20. save stone in the distance.

A bit later on you find the 54. big Eitrium cell on your way on the ground.

Directly after you will have to jump over a gap, here is the 55. big Eitrium cell, as well as the 26. audio log on the ground.

After another gap you can collect the 56. big Eitrium cell.

Shortly after will be the 57. big Eitrium cell in your way.

A bit later, go right, jump down. There will be the 58. big Eitrium cell, as well as the RFID key 6 at the body.

Shortly after you find the 14. small Eitrium cell on the right, before entering the next tunnel.

After jumping over a few rocks, on one of them is the 59. big Eitirum cell.

On the next rock will be the 60. and on the next one the 61. big Eitrium cell.

In Helheim.

The 21. save stone will shortly be in your way on the left.

Later on will be the 22. save stone in front of you.

Back in Midgard the 27. audio log will shortly be in your way on the left on a table.

Before going into the container turn right. Here in a very dark corner will be the 24. orb. (You can max out the gamme here to see better which I also did. We don't have our night vision for now sadly.)

The 23. save stone will shortly be in your way. (There will also be a level 8 door here, as well as the RFID key 8, but we can't pick it up now, we will go back to this later.)

In the huge elevator, BEFORE ACTIVATING IT: go to the right of it to find the 62. big Eitrium cell.
(Again it is quite dark here, you can max out gamme here to see better.)

From there, continue on the right side till you find the 63. big Eitrium cell.

On the other side of the room will be the 64. big Eitrium cell.

Now go the left of the room to find the 65. and 66. big Eitrium cell.

Now go to the middle of the room where the console is to find 5 more big Eitrium cells, so we are at 71 (got all I found now).

Directly after the elevator ride you find the 23. save stone.

As well as the 28. audio log (got them all now) and the 16. energy machine.

After getting the Jarngreipr back, go back to the West Wing. After you took the huge elevator again, take the RFID key 8, to the right of the save stone, next to the container in some blood.

Directly from there, go in the level 8 door, which is just a few steps on the left. Inside will be the 15. small Eitrium cell (got them all now).

After the container section with enemies, instead of turning right, go straight, open the door with the terminal and fine the 25. orb inside.

Back in Helheim.
Next to the save stone we found earlier there are the 17. and 18. energy machine. (Yes, I missed those earlier.)

Continue and find the 24. save stone on the left, before the big gate.

As well as the 19. and 20. energy machine shortly after.

In the cave, at the start is the 21. energy machine.

There, take the first left and then left again to find the 26. orb.

Go back up the stairs and take the next door on the left.
Shortly after you find the 22. energy machine.

From there, go right, up the stairs and turn right again to find the 27. orb.

Go back to the energy machine, turn right and continue. Shortly you find the 23. energy machine.

Continue on a bit, then take the second left to find the 24. and 25. energy machine.

After the big gate and after a door will be the 25. save stone.

From Hel's body, turn right and find the 28. orb behind a pillar (got them all now).

Dealer of Death
Use large pulse of Jarngreipr on 30 creatures
Just kill 30 enemies using a blast of the Jarngreipr (charge a full shot and then release)


Good choice
You chose wisely
In the tutorial, after getting your new eye, when you have to choose a rune, choose this one


Second chance
Get out of the ether
If you die, you go to the ether first and have to place two balls in the orange circles. An enemy will patrol there. Only if he gets you, you will really die. If you get both balls in the circles, jump in the middle green hole to get back to life and get that achievement.


Explorer
Enter all rooms in ”Apsulov Research”
You get to Apsulov Reserach in Chapter 2, but as you need the RFID keys 1,2,3,4,5 and 7 you can only get this in chapter 4. You will get this achievement if you follow my collectible walkthrough.


I like it here
Go to the ether 10 times
Just go to the ether 10 times. You don't even have to get back to life for this one.


Paranoid
Save game 50 times
Just save 50 times at any save stones. You can if you want just save at one save stone over and over till you get this.


Family first
Join your family in ruling the new universe
One of the two endings. For this one: join Loki (choose Lokis hand).


I rule alone
I will rule the new universe alone
One of the two endings. For this one: destroy Loki (choose Henriks soul stone).


Completionist
Complete all achievements
Just get every other achievement for this one.

The speed of Thor
Complete game on any difficulty in under 3 hours


The speed of Odin
Complete game on any difficulty in under 2 hours.



Those achievements will pop up as soon as you choose the ending if your time is under the time limit

unclear stuff
-I don't know if the time stops when you pause the game, better not do it or do it at your own risk
-I don't know what happens when you reload a checkpoint, but it might not reset your time, but it maybe it does?
-maybe loading screens or unskippable cutscenes do not count towards your time ?

Tips:
-play on New Game + while having collected all 15 Eitrium cells (the little ones) in your initial run, so you don't have to worry much about those in the speedrun; also choose EASY difficulty (Seer)
-ignore everything which isn't relevant, so all collectibles, even the big Eitrium power cells you don't need, HOWEVER you can collect some of them without loosing time because some are directly in your way and you can collect them even while running; you can use some big power cells in the boss fight to save you some time and also at some key moments you can use some while you have to wait anyway, so you don't have to recharge your cells at a station or kill ghosts so often
(I only used them during the boss fight, but you can probably win some time by using them more often. In my run I ended up with 25 of them, using 17 of them in the boss fight, so yeah, you can use a few before that. Btw. I think I dealed quite bad with the boss.)
-only kill enemies when really necessairy, usually you can just outrun them
-always run if possible
-only recharge your energy when really necessairy
-don't let your night vision run out, so only use it when necessairy
-always know your way! (the speedrun should be the last thing you do in this game, you should have played through the game at least twice before doing the speedrun to make it easier for you)
-saving is not needed (waste of time), only save when you are locked into a room and have to watch a cutscene or so, but please don't die, it shouldn't be hard;
BUT you have to save at the first save stone once to be able to progress!!!
-when pressing TAB the game continues, so the time will NOT stop, luckily you never need to press TAB
-in the tutorial you will have to do two out of three possible tasks (using a code, using a key which is in a box or find a button which is under a little cube), the ones you have to do is always random, the order as well
-write down all the codes so you don't have to look for them ingame; I'll tell you all here, but better write them down on a paper next to you, you don't want to open the steam overlay during the game, also know where you get the RFID keys or write it down (I will tell you that as well)


Codes

Chapter 1
-tutorial: 1436
-6725
-6725
-elevator: 7913

Chapter 2
-3425 (first go to Henrik before using this, I think you can't skip a part here)
-4673 (you can skip a big part using this right away)
-East Wing: 3825

RFID keys which are (a bit) hard to find:
-2: after dropping down from the roof, it's in the second locker on the left, in the same one where the skull key is; you first have to get the skull key, then you have to scan the RFID key
-3: in the morgue, after dropping down from the vent, go down the stairs, go the other stairs up, turn right and scan the last body
-4: facing the level 4 door you have to go through: go left into an area with 4 rooms, go into the second one on the left and blast the enemy, then scan his key


Speedrun Video Walkthrough
note: on my first speedrun I got about 1:45, on my second I got 1:41, on my third one I got 1:37. I still made some small mistakes, maybe it's possible to save 1 or 2 more minutes, but not much more I think. So the room for mistakes is about 23-25 minutes. Should be enough.
Although keep in mind I played quite agressive/risky in my third run, by just running by enemies, getting hit often and also getting some fall damage here and there. If you don't want that, it's all right to play a bit more safe. On my second run I played quite safe.

agressive/risky: https://www.youtube.com/watch?v=Yo591BcejrY
more safe: https://www.youtube.com/watch?v=U2CYAntoUcE&t