The visuals and music are awesome and the controls feel good, my only feedback for as much as I've played of it are: it would be nice if some clearer sense of progression was implemented, the more complex attacks and the environmental changes are cool, but I never felt as if I was really doing anything but pissing off the boss as opposed to actually hurting it. Second and last, which may just be my personal preference, the player agency doesn't really feel all that satisfying to me, the flow of combat is wait for recharge, wait for a blockable attack, block, resume waiting and repeat. I understand this is a rhythm game but I never really felt as if I was doing much besides waiting. Giving the player a bit more to do or think about besides dodging while waiting could go a long way to improve this sense of agency.
These are all just my opinions and I am not a game designer in any way whatsoever, so take this as you will. The game still looks great and plays really well and I can see that it has a ton of potential to be an awesome boss rush! Best of luck on your kickstarter!