This achievement is unlocked by completing the game on Hard difficulty. It's highly recommended to have played through the game at least once on Normal difficulty before you try Hard.
The big differences between Normal and Hard are as follows:
- Enemies will kill you in a single hit.
- There are fewer lamp oil and tinderboxes available to find.
- There are no autosaves or checkpoints.
- Manual saves costs 4 tinderboxes.
- You can die from your sanity going too low.
Amnesia: The Dark Descent is not a very difficult game, especially if you've already played through it before. However, some parts are still challenging, and become more so for the Hard difficulty. Particularly any area with enemies, since they now insta kill you.
Tips for getting through Hard mode:
- Speed is your friend! If you're quick, then there will be less time for your sanity to go down, and shorter between each time you receive a bonus to your sanity for completing objectives. That said, do have some patience for waiting for patrols to move away, as rushing through areas with monsters can get you killed.
- Never use tinderboxes for lighting any light sources! Use your oil lamp a lot instead.
- Be very familiar with the game, and the location of items! That way you won't waste precious resources exploring.
- Since you don't have limitless saves - only a small handful, in fact - make sure to save only when necessary. I suggest saving only before areas with monsters, where there's a big risk and chance of dying involved. Personally, my first save was after finishing the entire first area, and using the acid to get through the meaty spiderweb thing.
- Don't give up! Sometimes you're gonna have to do things over again, but you will get through it eventually, and get far enough that you'll be able to make a new save.
Good luck!