Complete every single one of these requirements while playing to unlock the costumes and once you have them all you will unlock this achievement.
Battler: Jack 35 Battlers.
Bratgirl: Defeat 150 Brat Girls.
Doom Monkey: Defeat 500 Doom Monkeys.
EE-Lectric: Jack 50 EE-Lectrics.
Goar: Jack 30 Goars.
Halloween: Successfully complete 50% of the game.
Koo-ala: Defeat 150 Koo-Alas.
Magmadon: Jack 50 Magmadons.
Ratnician: Defeat 200 Ratnicians.
Rhinoroller: Jack 30 Rhinorollers.
Scorporilla: Jack 15 Scorporillas.
Shellephant: Jack 15 Shellephants.
Sludge: Jack 50 Sludges.
Snipe: Jack 50 Snipes.
Spike: Jack 50 Spikes.
Stench: Jack 50 Stenches.
Valentine: Collect all Gold Idols.
Voodoo Bunnie: Defeat 200 Voodoo Bunnies.
Thanks to cheatcc.com for the list.