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.
Complete Slayaway Camp X "Space Camp"
Unlock 13 Killers using the Mystery Box.
Unlock the gorepack called Cinco de Slayo.
Complete Slayaway Camp 8 "Manhattan Manslaughter"
Unlock the gorepack called Puffed Up.
Unlock the gorepack called Party's Over.
Unlock the gorepack called Charbroiled.
Unlock the gorepack called Face Stapled.
Unlock the gorepack called Demonized.
Unlock 13 Gore Packs.
Unlock the gorepack called Grenade!.
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.
Unlock the gorepack called Not the Bees.
Unlock 5 Killers using the Mystery Box.
Unlock the gorepack called Origami'd.
Unlock the gorepack called Double Blind.
Get a score of 20 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 5 in Faces of Killed 3.
Complete Slayaway Camp 7 "Office Hours"
Unlock the gorepack called Pruned.
Kill a victim by scaring them into an electric fence.
Complete Slayaway Camp 6 "Fatality Ward"
Kill 30 dumb cops across all Slayaway Camp movies.
Unlock the gorepack called Spronged.
Kill a victim when the lights are off.
Complete Slayaway Camp 5 "Bloodbath Beach".
Kill a swat cop.
Use the Mystery Box once.
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".
Unlock the gorepack called Broken Heart.
Complete Slayaway Camp 2 "Return to Slayaway Camp".
Kill a victim by knocking over a fridge onto them.
Unlock the gorepack called Pincusioned.
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 666 victims across all Slayaway Camp movies.