The game design here is very good. Good telegraphing, varied attacks. The final boss was a cool mashup. There are a couple of shortfalls. One is that due to the slow chipping away of the boss' health, the fights feel longer than they really are. I also felt the character had too much health, since at some point in every fight the intensity vanished as I knew that even if I was hit several times in a row, my victory was assured due to consistent damage output. Further, the beginning of a fight was never tense as I had a comfortable amount of health. Upon reflection, I think this is why many games have healing mechanics (or just give you lower health and a shorter fight) instead of one giant health pool - to ensure tension is maintained no matter how well you do.
Sound: Musically it was good and did the job, but the mixing between sounds, and in the music itself, was poor overall. At times the music made sound effects inaudible. When you make sounds and music, try to consider how they'll sound in tandem as you create them - try not to create any sound in isolation and consider it as part of the broader soundscape.
Visuals: Very basic but again, they achieved what they needed to. Refinement will come with time. My favourite design was definitely boss 2.
Anyway, very good submission