This achievement requires that you purchase all of the outfits from the shop. The shop can be accessed by tapping the shopping cart symbol from any level-selection screen. Once in the shop, select "Outfits" at the bottom. There are 10 outfits you need to purchase using coins you have earned. The total cost of all 10 is 7,400 coins. You should save this achievement for last, as outfits have no gameplay implications and are entirely cosmetic. See the "Best Coin Grinding Method" section at the end of the Roadmap for the quickest and easiest way to earn coins. As soon as you purchase the last outfit, the achievement will unlock.