Kept me hooked! By the third song I was wondering if selecting the highest difficulty was a mistake, but grinding all the upgrades got me to victory! (weirdly the boss went down in 1 round when some others took 3+ tries). At one point I had to close and reopen firefox because it was getting kind of choppy/laggy, probably just had too many tabs open, but it's strange that it started out smooth, maybe some of the later songs take a bunch of memory?
Loved the characters, good challenge, and lots of fun, great work!