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