After 3 total hours of playtime, all chests have a 2.25% base chance to instead spawn as a mimic, which increases by 2.1% for every point of curse up to a maximum of 25.35% overall. An easy habit to get into is upon finding a chest, shoot it 3 times with the starting gun of your gungeoneer, eventually you'll encounter a mimic and get the trophy.
If you're in a rush, you can always boost this by inflicting curse upon yourself with curse jars, shrines, and Cursula to raise the chances significantly but if you're already going for the platinum you're bound to run into tons of mimics by the end.
Cheers.