Here is the full list of all 31 Skeletery achievements.
Doot the skeleton.
Complete level 1.
Complete level 2.
Complete level 3.
Complete all levels.
Collect first doll.
Collect all dolls in Level 1.
Collect all dolls in Level 2.
Collect all dolls in Level 3.
Kill first human.
Kill 10 humans.
Kill 30 humans.
Kill 70 humans.
Kill 100 humans.
Place first building.
Place Large Tombstone.
Place Warrior Crypt.
Place Fountain Garden.
Place Skeleton Crypt.
Place Cursed Graveyard.
Place Coffins Carts.
Place Statues Clearing.
Place Dogs Graveyard.
Use Extra Life Buff.
Use Attack Damage Buff.
Use Currency Regen Buff.
Use Death Ray Buff.
Use Extra Health Buff.
Use Health Regen Buff.
Use Skeleton Dogs Buff.
Use Skeleton Hands Buff.