A simple guide to get all the achievements.
Pacify Achievements
Start a fire!
Light a fire in the boiler. For this you will need wood and matches, both of which are in the basement boiler room (opened by the key in the attic room).
Burn a Doll
Burn one of the marked dolls (one of them is in the basement on a white table, going down the outside hole to the left and then to the right of the large room with vases. The rest are enclosed in the attic and will run as soon as you open it, spreading throughout the house).
Wrong Doll
Burn one of the normal dolls (they are scattered around the house).
Key Master
Collect all keys of the house in single-player or cooperative mode. The order is: living room, bedroom, basement, attic and main door. Once you find the key to the living room (it can be on the basement table at the bottom of the staircase next to the boiler room. Or on the ground floor on the landing unit just up from the basement, or in the yard bench, or on the little table in the small room on the left where the staircase leads to the first floor). The rest of the keys are found consecutively. The bedroom is in the living rooms or kitchen and the attic one in one of the bedrooms or bathrooms. Finally the last two keys always appear in the same place. The basement key is in the attic room, and the main door key is in a tiny room in the basement under the staircase leading up to the kitchen. This fifth key is not necessary to pass the game because, once burned the last doll, the main door opens automatically.
Key Hoarder
Same as the previous achievement but in versus mode.
Pacify Her
Give a normal doll to the girl when she is in monstrous mode (to do this just wait a few minutes for her to transform or burn a marked doll and she will be transformed right after).
I love lore!
Read all the notes in the game.
Bully
In versus mode, push another player (when in front of you, press the main mouse button).
You are a doll
In cooperative or versus mode, let the girl attack you when is in monster mode.
One Man Team
Win the game in single player mode. In other words, burn the 9 dolls and take the girl’s body to the van or burn it.
Win with Friends
Complete the game in co-op mode.
Destroy the competition
Win the game in versus mode (you have to burn more dolls than the other players).
Save a friend
Give a normal doll to the girl when she is attacking another player (do this before she turns him into a doll).
Revive yourself
In cooperative or versus mode, when the girl has attacked you and you are a doll, burn yourself in the boiler (if there is no fire, in cooperative mode you will need someone to start the fire for you. In versus mode you will be able to start it by yourself).
Beat the game developer
Complete the game in less than 20 minutes.
Burn the girl (Secret Achievement)
It’s the only hidden achievement and it’s about burning the girl once you’ve burned all the dolls (you have to find her body around the house and instead of taking it to the van, burn it in the boiler). This will unlock an alternative ending.