Really interesting concept, has my mind going all kinds of places with possibilities. What you've executed here is good, nice animation work for both the boss and player and the art style is really pleasing to me. I think the stun state is perhaps too long, the loss of control for that length of time before having to dodge the next attack felt a little but repetetive after happening a good number of times (perhaps I'm not very good though lol).
The added elements when progressing through boss stages is great, the arrival of the skeleton dudes gave me another dimension to think about.
I thought the controls worked well too, I didn't understand right click until I got the bosses health to 0, and the game helpfully reminded me to use it, which was fantastic. The tutorialisation was well executed on that front. You should be very proud of what you've made :)