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