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 every Costume Ability in battle.
Collect all Creepy Treat Cards.
Acquire the Pumpkin Costume.
Win all prizes in Bobbing for Apples.
Defeat Metxel in battle.
Acquire 5 Costumes.
Win the Autumn Haven Mall Costume Contest.
Get the jump on 10 different monsters by pail bashing them to initiate combat.
Defeat BoJonn in battle.
Collect a total of over 4,000 candies.
Defeat a monster in combat with a friend by your side.
Use the Robot Costume to Boost off of 15 different ramps.
Beat the game.
Complete all Quests in the game.
Collect all Battle Stamps.
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.