Really cool project!
I really like the idea of changing the music based on the phase of the boss, I thought it was a very cool idea as the music evolves throughout the fight.
I feel like this game could use some "game juice" relatively small stuff that would make a big impact and enhance the experience. Things like camera shaking/screen freezing once you get hit, and some feedback on hits in general.
Also, it was a bit difficult to damage the boss during it's sun attacks, although I'm sure partially that is just a skill issue on my end.
All in all, some sprucing up and you have a great project!