Successfully complete one of the following tasks to get a trophy:

Battle Buds (Bronze): Defeat a monster in combat with a friend by
your side.
Binary Bouncer (Bronze): Use the Robot Costume to boost off of
15 different ramps.
Do the Monster Bash (Bronze): Get the jump on 10 different by pail
bashing them to initiate combat.
Dressed to Quest (Bronze): Acquire 5 Costumes.
The Last Gourdian (Bronze): Acqure the Pumpkin Costume.
Downsized! (Bronze): Beat BoJonn in battle.
Dozer Dodger (Bronze): Defeat Mextel in battle.
Master of Disguise (Silver): Use every Costume Ability in battle.
Sweet Justice (Silver): Complete the game.
All Decked Out (Silver): Collect all Creepy Treat Cards.
They'll Be Worth A Lot Someday (Silver): Collect all Battle Stamps.
Mask-O'-Raider (Gold): Complete all Quests in the game.