Excellent all around.
Only have 3 very tiny nitpicks:
1. Screen resolution is super tiny...i played it in full screen and it was mostly fine however.
2. Would be nice to have something besides a flat background color. The sprite work and animations were great otherwise.
3. For some reason (maybe it was just me), the boss healthbar obscured the bottom of my screen - making it hard to see the player character and enemies on the ground. That could be due to my monitor resolution though as I use an ultrawide for productivity.
Overall, great game, very fun and satisfying. Cool implementation of the theme.