To get this achievement, set up a local game between yourself and a guest, or a second profile. Set it to the "Classic" map, "Battle" mode, with a 2 minute time limit, and 9 rounds. Then, destroy as many soft blocks as you can, so that the clothing item does not spawn on your guest/second profile. Wait until "1:00" comes round on the clock (one minute after the start of the round), and you'll see a shining, spinning icosahedron (a 20-sided shape made of triangular sides) item appear somewhere in the arena. Pick it up. One of these seems to reward you with 2% of your closet, so logically, there can't be more than 50 costumes. Only one item appears per round. The icosahedron is immune to bomb blasts. Once the item is collected, kill the guest/second profile or wait until time runs out, and the round is declared a draw. Repeat until you have the achievement.