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