Before we start, there are a few things to mention:

• There are 33 achievements in total, split into 4 categories below.
  
• Cheats don’t disable achievements.
  
• You can select any level from the main menu, and achievements will still work.
  
• Some achievements are slightly bugged but still obtainable.

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There is an achievement for every gun from DOOM and DOOM II:

Guns Are For Wusses
Get 25 kills with your Fists in a single level.


This stacks really good with the A Man and a Half achievement, check it out in the Challenges section of this guide.

Until It Is Done
Get 100 Chainsaw kills.
Update (Oct): Looks like it was fixed and chainsaw kills are finally getting counted in DOOM I as well!

Shoot It Until It Dies
Finish off a Cyberdemon with the Pistol.


Start level E2M8 (DOOM I) on any difficulty, pick up the rocket launcher, and save after every couple of hits on the Cyberdemon. When it finally dies, load the game and finish Cyberdemon off with the pistol.

Boom Shaka Laka:

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Groovy
Get 3 multi-kills with the Shotgun.


Skeet Shooting
Kill 4 enemies in 1 shot with the Super Shotgun.
Note: There is no Super Shotgun in DOOM I & Sigil

Heavenly Joy
Shoot 200 bullets consecutively with the Chaingun.


Bowling for Gibs
Gib three or more enemies with a single rocket explosion.


Not just kill 3+ enemies at the same time, but leave nothing but a gooey paste from them:

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Cookin' With Plasma
Kill 5 enemies in 5 seconds with the Plasma Rifle.


Overkill
Kill only a single enemy with a BFG9000 blast.

There are 2 achievements based on new weapons from the Legacy of Rust expansion:

Kill It With Fire
Immolate a total of 30 arachnoid enemies with the Incinerator.


Screen Wipe
Obliterate 50 enemies with a single shot of the Calamity Blade.


Tip (by Steam User @Klobby):
"for screen wipe, in E2M6 you will get teleported after picking up yellow keycard into a room where 2 floors will rise up, each having more than enough imps to get the achievement"

There are 7 achievements tied to completing the last level of each game. If you're only after the achievements, you can simply select the final level from the main menu (just not secret level!), finish it (or press ~ and type exitlevel) and unlock them without playing through the entire game. But, come on, bro... Just play the damn game you bought!

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DOOM
Reach the end of 'DOOM'.
Single Player > Level Select > Episode Thy Flesh Consumed > E4M8: Unto the Cruel


DOOM II
Reach the end of 'DOOM II'.
Single Player > Level Select > Level 30: Icon of Sin

Master Levels
Reach the end of 'Master Levels for DOOM II'.
Level Select > Level 20: Mephisto's Mausoleum

Sigil
Reach the end of 'Sigil'.
Level Select > E5M8: Halls of Perdition

No Rest for the Living
Reach the end of 'No Rest for the Living'.
Level select -> Level 8: Tomb of Malevolence

Evilution
Reach the end of 'TNT: Evilution'.
Level select -> Level 30: Last Call

Plutonia
Reach the end of 'The Plutonia Experiment'.
Level select -> Level 30: "The Gateway of Hell

Legacy of Rust
Reach the end of 'Legacy of Rust'.
Level select -> E2M7: Brink

The rest are special challenge achievements:

Timing Is Everything
Kill 2 enemies with a single barrel explosion.


Can be done near the exit of the first map of DOOM I, E1M1:

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Doormat
Crush a corpse in a door.


Same E1M1, or basically any other map:

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Overprepared
Finish a level with 200% armor and 200% health.


My pick would be MAP07 from DOOM II. It’s a simple arena with 4 MANCUBUS(es) on platforms. After killing them, walls will open revealing 8 Arachnotrons that you'll have to deal with and that's about it... Before exiting, hit all wall levers to lower the platforms. One has a Megasphere that boosts you to 200 HP and Armor. Then, exit through the organic switch in the middle.

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Indiscriminate Headhunter
Kill one of every enemy.


There are 23 enemies to be killed, secret level nazis don't count, but new demons from Legacy of Rust do. Kill Them All:

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Not So Friendly Fire
Cause an enemy to kill another enemy.


Let them Bang (most likely it'll just pop up by itself during your very first playthrough):

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More Like a Dream
Complete a level on Nightmare difficulty.


Run through DOOM II first level, like this:

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Hoarder
Finish a level with 100% items picked up.


The Only Thing They Fear Is You
Complete any monster-filled level with 100% kills without taking any damage on Ultra-Violence or higher.

Tip (by Steam user @Lenyn4K)
"Do it on the Map 01: Entryway from DOOM II:
1. Kill all the enemies in the map. Doesnt matter if you get shot, just keep forward and be sure to kill every single enemy in the level.
2. Take medkits and be sure your health is 100.
3. Get in front of the exit and press F5 (savegame bind).
4. Exit the level.
5. At the start of the next level, press F9 (Loadgame bind) it will automatically load the previous saved game.
6. Now you will be in front of the last level again. Go ahead and exit once again.
7. Profit."

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An Important Looking Door
Find a secret area.


Alternate Dimension
Find a secret level.


Easest way of doing it would be E1M3 level from DOOM I, here's <1 min the video:

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Clean Slate
Kill all monsters in any monster-filled level on 'Hurt me plenty' or higher.


Burning Out of Control
Complete any monster-filled level with 100% kills, items, and secrets.


A Man and a Half
Get 20 kills using the berserk powerup in a single level.


Can be easily done on E2M2 from DOOM I, there's a berserk powerup right at the beginning of the level:

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Untouchable
Complete 8 different levels sequentially under par time without dying on Ultra-Violence or higher.

There are 4 ways of doing it:

1. I'm Too Young to Die: just type exitlevel in console 8 times.
   
   
2. Hurt Me Plenty: Tip (by Steam user @erc):
   "Untouchable can be unlocked with the 'No Monsters' modifier enabled. Just got it in a E1M1 - E1M7 run. You should also visit E1M9, of course, since E1M8 can not be completed with this modifier."
   
   So the order of the levels should be like this:
   
   E1M1 > E1M2 > E1M3 > E1M9 > E1M4 > E1M5 > E1M6 > E1M7
   
   If you don't know how to get to the secret level (E1M9) check out Alternate Dimension achievement description above!
   
   
3. Ultra Violence: The good old fashioned honest grind - here's the video of me doing it the Right Way ♂ (without modifiers), I suggest you trying doing it like that, it's A LOT of fun but can be pretty challenging:
   

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4. Nightmare: Same as Ultra Violence: but on NIGHTMARE difficulty. Can you do it?