Really fluid movement and juicy mechanics! I really liked the small text cutscenes when the boss went into overdrive. I was able to beat the current three bosses fairly easily so maybe an increase in the last boss difficulty for your next update to the demo. Some things that would make it juicier:
Add some particles/bloom on the player when he has sword charge.
Some more SFX and VFX for some of the enemies movements and attacks.
But that's just a limitation of time and game jam games. The camera movement was cool too and added to the juiciness of the fight. Great game overall!