This guide will eventually help you get 100% of the achievements. There are spoilers bellow, so i'd suggest at least completing the game once before reading it.
How to Obtain All Achievements / Trophies
- Awoke with fragmentary memories after the crash of the Cassandra.
- Followed Salim's trail.
- Dreamed of Paris.
- Carried on with life, despite Salim's fate.
- Followed the pilgrim's trail to take refuge in the fort.
- Explored the inner fortress, and spoke to the doctor.
- Took time to talk with an unborn child.
- Fired a cannon at the fortress gate.
- Found a way through the ruined cistern.
- Travelled through the desert.
- Took a walk in the park.
- Escaped the Shadow.
- Activated the alchemists' device.
- Drained the fear and the life from a crewmember.
- Activated a portal between worlds.
- Operated a scanning device, and learned of a child's illness.
- Failed to reach a child, separated by deep waters.
- Reached the doctor.
- Gave birth to a daughter, only to lose her.
- Completed the Story.
Popped the lid on some carefully stored meat:
When I got to the water part of the cistern, towards the beginning of the water-filled tunnels I opened a door that was up high on a wall which had a blue wheel below it that you can turn. I turned it, opened the door, and got the achievement.
- Sent extra objects through the portal in the Alchemist's Chambers.
- You're required to send a specified number of objects (I guess between 6-9) through the teleporter in the lab located in the other dimension. Behind the teleporter is a shelf with more than enough stone cubes to choose from.
- Resisted fear, and resisted anger.
- Based on what a few others have said and how I assume it worked for me to unlock, you don't want turn into a ghûl while facing the empress. So you'd want to do either the iconoclast or Provider ending to and die as less as possible during the campaign.
- Escaped a cage in the Arsenal without setting off a grenade.
- While fetching the sulfur solution for the tank shell composition. Enter the cage by jumping across the scaffolding outside the window next to the cage, this lets you enter the cage without triggering the grenade. Get all relevant items in the cage and fetch the key from the guard leaning against the cagebars (holding it in his hand). (Carefully) remove the hook from the door and unlock the door with the key.
- There is also a way to escape without touching the rope or key - when you get inside the cage look for a crack in the wall on the side with the medical cabinet. You'll be able to break it open and escape.
- Smashed lots of irreplaceable objects.
- Smash a set amount of vases. I would guess it can be as many as 20.
- Tortured an innocent to aid an unborn child.
- Create the vitae as shows in the video and then use it in the chair.
- Spared an innocent, perhaps dooming an unborn child.
- Power up the cart and leave without creating the vitae.
- Gazed at a reflection in the water.
Outside the fortress, move around the left of the building. There is a little section containing a fountain. Interact to gaze into the fountain
- Burned a forest's worth of matches to illuminate dark places.
Requires 120 individual matches to be burned. Lighting things (such as candles and lanterns) with those matches doesn't affect the number that's burned.
- Kept quiet and still, and so escaped a predator's gaze.
- Go prone (hold CTRL with standard keybinding) near an enemy without being detected. The best places to do this, is either in the factory just after you pick up the power triangle, or in the lower factory after you fall off the cart.
- Left when things were getting interesting in Amnesia: Rebirth.
- As you enter the caves. After reading Salim's letter and walking a short bit into the dark hallway behind it, you can save and exit the game to quit to unlock this achievement.
- Completed the game faster than any Frictional developer.
- You don't really need to watch the full speedrun, although you can follow it along. The best thing would be to give from 51:00 - 1:05:50 a watch before attempting it. This is where I use a door glitch in the alchemist lab, this allows you to bring 2 Layden Jars into the portal room and completely skip having to go and find the second Layden Jar, which you'd normally not have at this point. Doing this glitch also makes the ending faster as well. After riding the elevator up to the throne room, you can just go straight through the portal as you should still have a one Layden jar.
- Collected all the other achievements.
For these achievements you can do them one after another by clicking continue when going back to the main menu after finishing the story.
- Escaped to Paris with a daughter that must now be cared for:
- Once you've soothed Amari after picking her up from the crib, take the Leyden Jar before exiting the room. When the boss fight starts, Go to the door use entered from, using the structures to avoid her gaze. Then use the Leyden Jar to power the portal
- Left a daughter in the care of a god, and was reborn.
- Once the boss fight starts, put Amari back in the nursery.
- Sacrificed everything to cast down a god and end suffering:
- Once you've soothed your baby after picking her up from the crib, take the Leyden Jar before exiting the room. When the boss fight starts, go to each of the rooms around the side, starting with the first on the left (whilst avoiding the spirits gaze). Use the Leyden Jar to open the doors and then inject the valve.
- Found treasure beneath the water.
- Located at the desert oasis right after traversing a riverbed in the desert. In the middle of the oasis is a rock. Underwater and near that rock is some old pottery. Hidden in that pottery is a can of oil which triggers the achievement.
- Collected notes to fill a journal.
- Burned a forest's worth of matches to illuminate dark places.
Crewmember, Historian & Curator
- Found all of the crewmember cards.
- There's 14 crewmember cards you have to collect throughout the campaign. How to complete it see this guide.
- Found an unfinished letter from an archaeologist to his sister.
- The historian achievement is triggered in the Fort. While searching for a way to the radio in the house, on the same floor (2nd) as the cannon which is needed to break the floor are some stairs leading up to a well lit corridor. Going up the stairs and taking a left at a junction, you will traverse another flight of stairs and see a covered box. In said box is an unfinished letter. Reading it unlocks the achievement.
- Found all the memories recorded by others. See this guide how to complete.