After reading Leebbullough's solution and Surferhead20's solution, I started wondering what the actual minimum requirements were for this Achievement. So much so, that I gave it a try! I combined information found in both solutions to see what the MINIMUM requirements are for this Achievement, and this is what I came up with...
MINIMUM REQUIREMENTS:- Do NOT break ANY totem
- You need to have the "pacifist" playstyle, meaning you're not allowed to kill monsters
with your flashlight
- Do NOT kill the sheriff in Chapter 17, rather, walk off, through the door
- Do NOT kill the lumberjack in Chapter 17, rather, walk off, through the door
Irrelevant:- Doesn't matter if you pick up dolls or not
- Doesn't matter if you pick up pictures or not (excluding the ones in totems of course)
- Doesn't matter if you make any phonecalls or not
- Doesn't matter if you read any text message or not
- Doesn't matter if you choose to leave Bullet or not (Chapter 14)
- Doesn't matter whether you obey Carver when he gives you orders to stand still, crouch and stand up, or not
- Doesn't matter whether or not you send Bullet under the tree
The Achievement should pop after the final cutscene. This playthrough/playstyle should also give you the "Pacifist" Achievement if you haven't already unlocked it.
Here's a video tutorial on how to get this Achievement: