Nice, I enjoyed playing! I love the visuals for this game and all the boss designs are very cool.
It takes a few tries to learn the timing and visual tells for boss attacks, but you get into it pretty fast. The attacks could additionally be telegraphed with sound cues for better clarity and perhaps make a brief animation just before the attack so the player knows when to dodge. That way you can easily spice up the boss variety by having them do different attack patterns, like consecutive attacks or something. Also, not sure if I missed it, but it should be explained that the mouse position determined where you punch, since I just kept punching nothing for a while because my mouse wasn't on the boss.
Very nice game otherwise!