Once you acquire your 5th costume, presumably in the mall since you'll get the 5th there if your going for all the costumes and all achievements, then this achievement will pop.
The following are all of the costumes acquired from the game, just 5 are needed for the achievement.
From OG T UNIT, @ xbox360achievements:
- Robot: Start the game with this costume
- Knight: Save Everett from the Mummy bullies in Auburn Pines
- Statue of Liberty: Pattern and Materials from the Patriot kids in Auburn Pines
- Space Warrior: Pattern from the Autumn Haven Mall security guard and the materials from the friendly workers in the closed section of the mall.
- Unicorn: Pattern from winning the Costume contest in the mall. The pieces are hidden throughout the mall. The hardest one to find entails you falling through the railing of a higher level.
- Ninja: Pattern after collecting the 3 horns for the Monster Slayer Kids (Ninjas) on the 2nd level of the mall. The Materials are found scattered through the 1st floor of the mall. Talk to the kids to get them.
- Pumpkin: See the LAST GOURDIAN ACHIEVEMENT below. Each material piece is found by talking to the scarecrow on the 3 different stages.
- Vampire: Pattern from the carnival ticket-taker when you enter the fair. Materials are found in coffins around the same area.
- French Fries: Get this from Chip at the French Fry stand North of the carnival entrance.
- Black Cat: Get this costume as a reward for collecting all the Creepy Treat Cards.
- Grubbin: Get the pattern and the materials from the Grubbins in the corn maze. Enemies won't fight you while wearing this costume.