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