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.
Defeat Araxia in battle.
Complete all quests in Repugia.
Use the Pirate hook on 5 unique ziplines.
Find the mysterious visitor in the cliffs of Repugia.
Acquire the Eyeball costume.
Beat the game.
Complete all Quests in the game.
Collect all Battle Stamps.
Use the Robot Costume to Boost off of 15 different ramps.
Acquire the Pumpkin Costume.
Collect all Creepy Treat Cards.
Win all prizes in Bobbing for Apples.
Defeat Metxel in battle.
Win the Autumn Haven Mall Costume Contest.
Acquire 5 Costumes.
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.
Play Costume Quest on Christmas.
Defeat a monster in combat with a friend by your side.