Hey there, I tried your game recently.
I think it's cool I can tell you tried to implement a couple different attack patterns and have multiple phases in the boss fights to make them interetsing. However, I think at some points there are times it became stale. Usually games can make boss fights more interesting by introducing more attack patterns or increasing the attack frequency when the boss is at lower health. With the 2nd phase maybe you can make the boss do 2 attacks at the same time or something.
Another thing is that it wasn't clear to me that you're supposed to stomp on the boss's head in the 3rd phase. I just continue stomping on the fish and heart but the health of the boss went negative and I couldn't figure out why I can't defeat the boss.
Also overall, it's obvious the game needs more work with the art, animation, sound effect, etc. But since this is a prototype for a single boss fight I think it's to be expected.
Overall cool game.