The base game of La-Mulana 2 has 46 achievements (the other 5 are for the DLC). All of them can be acquired in a single playthrough. Some of them (All I Need Is One Hand and All I Need Is One Finger!) can become missable and require a second playthrough. Otherwise, everything can be acquired in a single playthrough on Hard Mode.

Each achievement has a bolded line for the title, a line below the title for the achievement's description, and then my comments below that.

These achievements are ones you are required to get in order to complete the game, excluding the bosses. They are roughly in the order you'll likely acquire them, but the exact order will depend on where you go and what you do. I'm including where these achievements are acquired, as well as what items/progression are directly required, which are hidden in spoiler text.

Memories of Ruins
Obtained the Holy Grail .
This should be the first achievement you get, you pick this up early on in Gate of Guidance.

Knowledge is Power
Obtained the Ruins Encyclopedia app..
This is given to you in Gate of Illusion.


Corridor of Blood
Entered the Corridor of Blood.
Just like it says, enter that area. The first time you enter it, you should go from Valhalla.

She's Back
Met Mulbruk.
This will happen in Annwfn.

Grab a Wall
Obtained the Grapple Claw.
This is acquired in Icefire Treetop.

Lights of the Fairies
Got the cooperation of a fairy.
You'll get a message stating that the Endless Corridor is open at some point, going there you can meet Freya and acquire Freya's Pendant. Go to the Fairy King in Annwfn and you will get a fairy.

Running Like The Wind
Obtained the Gale Fibula.
This is acquired in Immortal Battlefield.

Seek a Higher Place
Obtained the Feather.
This is acquired in Dark Star Lord's Mausoleum. You'll need to go there through the Corridor of Blood and you'll also need the Mjolnir.

Ancient Alien Hypothesis
Obtained the Clay Doll Outfit.
This is acquired in Shrine of the Frost Giants, and you'll need to melt the ice and have acquired the Feather in order to get it.

The World Beyond Death
Finally reached the Underworld.
This is accessed through Corridor of Blood. and you'll need to chant some mantras to get there.

The Philosopher's Departure
Alsedana's funeral ended.
This doesn't have a specific time when it occurs, it's based on your general "progression" through the game. You'll get an e-mail about this when it happens.

Stop Time
Obtained the Lamp of Time.
This is acquired in Immortal Battlefield and you'll need the Cog of Antiquity to acquire it.

The Dark Lord's Nest
Were Led to Nibiru.
This is accessed from Dark Star Lord's Mausoleum and you'll need a couple items: Pyramid Crystal, Destiny Tablet, and Clay Doll Outfit.

The End
Saw Eg-Lana through to the End.
You'll need to defeat all of the Guardians, then fight Heimdall in Annwfn to get this.

Like Father, Like Daughter
Don't let that stop you from saving that pain-in-the-neck.
You'll get this in Immortal Battlefield after acquiring the achievement "The End". The Altar is the location you get it in.

These are the achievements for the game's 9 main bosses, plus one more for the final boss.

The order you'll get these in can vary. The first boss will always be the first one here, and then after that it's possible to do some of the bosses in different orders.

Dragon of Embraces
Defeated Fafnir. This is in Roots of Yggdrasil.


Obstacle
Defeated Vritra. This is in Valhalla.


Shouldered World
Defeated Kujata. This is in Annwfn.


Twined Malice
Defeated Jormungand. This is in Immortal Battlefield.


Insane Sun
Defeated Aten-Ra. This is in Dark Star Lord's Mausoleum.


Sky People
Defeated Anu. This is in Ancient Chaos.


Flaming Fist
Defeated Surtr. This is in Icefire Treetop.


Reproduction of Power
Defeated Echidna. This is in Hall of Malice.


Last Guardian
Defeated Hel. This is in Eternal Prison - Doom.


9th Child
Defeated the 9th Child. This is in Spiral Hell.

Moment of Release
Cleared the game.


Long Live the Queen
The Queen was saved from the ruins.
After defeating the 9th child and starting the escape sequence, go to Freya in Annwfn and talk to her. Then complete the game to unlock this.

You can activate Hard Mode in Mausoleum of the Giants.

Once You Read It, You Can Never Go Back
I warned you.
This is given for reading the Hard Mode tablet once. This will not activate Hard Mode on its own.

Dear Foolish Adventurer
Started HARD MODE.
Read the Hard Mode tablet a second time. You can get this achievement and then re-load a save if you don't actually want to play on Hard Mode.

By Sheer Grit
Cleared Hard Mode.

Watch Where You're Whipping
Received retribution.
Technically optional, but you'll get it at some point. Just whip something you shouldn't and you'll get it.

Take a Walk on the Dark Side
Obtained the Future Development Company app.
This is acquired in Valhalla.

Visiting the Underworld
Visit someone who knows of the Underworld.
Talk to Modgud in Icefire Trteetop.

How About a Makeover?
Found a Garb Chest that needs a special key to open.
Technically optional, but you'll see one in Cavern and you're required to walk past it at some point.

Master Key
Unlocked all fairy locks.
Use a purple key fairy to unlock everything that can only be unlocked by it. These are located in Takamagahara Shrine, Eternal Prison - Gloom, and Eternal Prison - Doom.

Pro Regular
Visited this NPC's store 20 times.


Everyone Has Their Reasons
Visited the store 40 times.
Steam censors the name of this NPC, but her store is in the village, just to the left of Xelpud. Just walk in and leave a bunch of times.

Brahma's Trial
Unsealed the Ruins Project Map
This is located in Ancient Chaos and requires a lot of work and puzzle-solving. Good luck!

Say Cheese!
Looking foward to the holiday snaps.
The cut-out shown in the image can be found in Gate of Guidance. Just stand in it for a second. Note that the positioning to unlock the achievement is very precise, so it might take a while to get.

What's the Wig Idea?
Helped the Elder overcome his weakness.
There's a statue in the village that you need to jump up and whip a few times to unlock this.

Harassment
Pepper is for food, not people.
Use the pepper on Heimdall in Annwfn.

These achievements require beating all 10 bosses (9 Ankh Guardians + the final boss) without using subweapons. You are allowed to use a shield, but not other subweapons.

Minibosses are allowed to be killed with whatever. Just the 9 Ankh Guardians + final boss must be killed without subweapons.

All I Need is One Arm
Obliterated all Guardians without using secondary weapons.


All I Need is One Finger!
Obliterated all Guardians without using secondary weapons in HARD MODE.

Serial Application
Obtained all applications.


Treasure Hunter
Filled every item slot.
Does not require getting all of the Garbs.

Encyclopedia Completionist
Unlocked every enclyclopedia entry.
You don't need to do this in a single playthrough. The Glossary available from the main menu tracks all entries across all playthroughs, so look there to see what you're missing.