Here is the full list of all 21 Costume Quest achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Use the Robot Costume to Boost off of 15 different ramps.
Acquire the Pumpkin Costume.
Defeat Metxel in battle.
Win the Autumn Haven Mall Costume Contest.
Acquire 5 Costumes.
Defeat BoJonn in battle.
Collect a total of over 4,000 candies.
Defeat a monster in combat with a friend by your side.
Get the jump on 10 different monsters by pail bashing them to initiate combat.
Use every Costume Ability in battle.
Beat the game.
Complete all Quests in the game.
Collect all Creepy Treat Cards.
Collect all Battle Stamps.
Win all prizes in Bobbing for Apples.
Acquire the Eyeball costume.
Use the Pirate hook on 5 unique ziplines.
Complete all quests in Repugia.
Defeat Araxia in battle.
Find the mysterious visitor in the cliffs of Repugia.
Play Costume Quest on Christmas.