Someone said this already but it's worth repeating: you get a lot more treasure chests on the first level, and treasure is based on your hero's level, so play that level repeatedly for equipment to use or sell. Also damage done by your special attacks does NOT count towards your levels, so if you're worried about the green guy leveling up to slowly don't use the charge attack (unless there are no enemies).
 "Bullet Heaven 2"){kind=icon}
 "Endless Zombie Rampage 2"){kind=icon}
 "King's Guard"){kind=icon}