OK. I want to say I hated this game ... until I loved this game!
It wasn't really clear from the tutorial that you could exit the screen and you would re-emerge on the opposite side of it. During the first boss battle I notice I jumped up from the top and ended up through the bottom, but thought that was maybe a glitch. It didn't help that the GUI was drawing correctly when I wasn't in fullscreen mode:
Then I realized I couldn't avoid the enemies raining blood across screen attack no matter what and discovered I had infinite jumps and could exit the left side and enter back in on the right side. Before that I was going to ask for an easy mode!
After I knew more of the mechanics, this was an absolute blast to play. Though I really wish the up arrow was jump.
I think if the third boss had actually been the first boss, players would have picked up on the ability to use the edge to their advantage (very clever use of the requirement by the way!).
Great boss states, nice varied attacks. Good flow and I loved the juice you did put into it. On boss two, I wish destroying his mini-monsters gave more feedback.
Overall though, I'm glad I kept repeating the initial boss fight because this ended up being one of my favorite games so far ... and this is my 70th game played and rated! Great work. Awesome solo dev effort.