Flipping the stage seems very creative and cool when it comes to planning how to dodge and damage the boss.
I really liked the art, the color is great! Just a minor note is that I think that when damaging the boss, there could be a little more visual or audio feedback to highlight the hit.
Very good use of the theme!
Something funny that happened to me was beating the boss at the same time he killed me. 10/10 xD