Costume Collector Trophy
Unlock 23 alternate costumes.
Once you've completed the game once and started your Legendary run, play up to the point where you choose sides. Obviously, you want to pick the side you didn't pick the first time through. Once you've done this enter the Simulator Challenge 'Basic Combat' and play until the infinite spawn switches. You now have infinite Fusion, Health and enemies to rack up kills for your costume unlocks! Character specific requirements for unlocking costumes can be found on your Stats page.