Best game I've played yet! It is original, and it really feels like the whole game is centered around this boss fight.
I noticed some aspects of the design that were really clever. I think the business of keeping upgrades on death was a great way to keep players from having to completely restart while still keeping the stakes of the battle high. I was not expecting that and it was really a nice surprise.
Two issues: The boss attack that leaves little x's on the ground doesn't seem to be dodgeable if you just happen to be there. Also, the idea that you get upgrades mid-battle from doing a certain amount of damage to the boss wasn't explained super well. When I got the dash, I instinctively clicked out before reading it thinking I had pushed a button that showed the tutorial.
I didn't finish, only up the the wizard stage, but might do so later.