Here is the full list of all 50 Slayaway Camp achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Unlock the gorepack called Pruned.
Unlock the gorepack called Origami'd.
Get a score of 20 in Faces of Killed 3.
Unlock the gorepack called Spronged.
Unlock the gorepack called Broken Heart.
Get a score of 5 in Faces of Killed 3.
Get a score of 10 in Faces of Killed 3.
Get a score of 13 in Faces of Killed 3.
Complete Slayaway Camp 2.5 "Another Return to Slayaway Camp".
Kill a victim by knocking over a fridge onto them.
Use the Mystery Box once.
Complete Slayaway Camp 2 "Return to Slayaway Camp".
Kill a victim by scarin them into water.
Kill a dumb cop.
Unlock 1 Gore Pack.
Complete Slayaway Camp.
Kill a victim by scaring them into a hole.
Kill a victim by scaring them into a fire.
Kill 100 victims across all Slayaway Camp movies.
Kill 666 victims across all Slayaway Camp movies.
Kill a swat cop.
Kill 30 dumb cops across all Slayaway Camp movies.
Unlock 5 Killers using the Mystery Box.
Unlock 13 Killers using the Mystery Box.
Unlock 13 Gore Packs.
Kill a victim with a Landmine.
Kill a victim when the lights are off.
Kill a victim by scaring them into an electric fence.
Get through a level with a cat without killing it.
Complete Slayaway Camp 3D "The Abusement Park"
Complete Slayaway Camp 4 "Slash'em High".
Complete Slayaway Camp 5 "Bloodbath Beach".
Complete Slayaway Camp 6 "Fatality Ward"
Complete Slayaway Camp 7 "Office Hours"
Complete Slayaway Camp 8 "Manhattan Manslaughter"
Complete Slayaway Camp X "Space Camp"
Unlock the gorepack called Grenade!.
Unlock the gorepack called Double Blind.
Unlock the gorepack called Face Stapled.
Unlock the gorepack called Charbroiled.
Unlock the gorepack called Demonized.
Unlock the gorepack called Party's Over.
Unlock the gorepack called Pincusioned.
Unlock the gorepack called Cinco de Slayo.
Unlock the gorepack called Puffed Up.
Unlock the gorepack called Not the Bees.
Unlock the gorepack called Juiced.
Unlock the gorepack called Ball'd Faced Lier.
Unlock the gorepack called C-flat Face.
Get a score of 25 in Faces of Killed 3.