Genetic Disaster (EP)

This achievement shouldn't be too hard to get. You can earn chests from mini-arenas, from defeating bosses and some other rooms on the different floors. If you're playing with more than one player, you'll get more chests from defeating the bosses and mini-arenas. I recommend playing up to the bunny boss on 3rd floor with as many controllers as you can, doing every mini-arena and defeating the bunny. This should get you tons of chests quickly.