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 5 Killers using the Mystery Box.
Unlock the gorepack called Puffed Up.
Complete Slayaway Camp 7 "Office Hours"
Kill a victim by scaring them into an electric fence.
Unlock the gorepack called Party's Over.
Complete Slayaway Camp 6 "Fatality Ward"
Kill a victim when the lights are off.
Kill 30 dumb cops across all Slayaway Camp movies.
Unlock the gorepack called Origami'd.
Complete Slayaway Camp 5 "Bloodbath Beach".
Kill a swat cop.
Complete Slayaway Camp 4 "Slash'em High".
Unlock the gorepack called Face Stapled.
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 Demonized.
Get a score of 25 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.
Get a score of 5 in Faces of Killed 3.
Complete Slayaway Camp 2.5 "Another Return to Slayaway Camp".
Unlock the gorepack called Cinco de Slayo.
Complete Slayaway Camp 2 "Return to Slayaway Camp".
Kill a victim by knocking over a fridge onto them.
Kill a victim by scarin them into water.
Kill a dumb cop.
Use the Mystery Box once.
Unlock 1 Gore Pack.
Unlock the gorepack called Not the Bees.
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.
Unlock 13 Killers using the Mystery Box.
Unlock 13 Gore Packs.
Complete Slayaway Camp 8 "Manhattan Manslaughter"
Complete Slayaway Camp X "Space Camp"
Unlock the gorepack called Broken Heart.
Unlock the gorepack called Grenade!.
Unlock the gorepack called Double Blind.
Unlock the gorepack called Spronged.
Unlock the gorepack called Charbroiled.
Unlock the gorepack called Pruned.
Unlock the gorepack called Pincusioned.
Unlock the gorepack called Juiced.
Unlock the gorepack called Ball'd Faced Lier.
Unlock the gorepack called C-flat Face.