Really enjoyable and overflowing with style! Like seriously, I love the aesthetic and the art is superb.
Honestly only a couple of things are holding this game back in my opinion. Here are some suggestions that I think would make for a more fulfilling experience:
Make it so that simply touching the boss does NOT hurt you, early on I thought its attack was triggering a jank hitbox until I realized I was dying cuz I barely touched it. I'm sure this was implemented to make one pay attention to their spacing, but it felt more unfair than anything.
The previous issue wouldn't be as bad with my next two suggestions if you wanted to keep the damage on touch. Firstly, make it so that the player character is always facing the boss, there's no utility in turning away from the enemy and makes it harder to space your attacks (also, it wouldn't make sense in a real combat scenario to turn your back on the enemy, which is more of a nitpick since it's a video game but seems appropriate for this type of game).
The other suggestion that could alleviate my first issue is to make the player's attacks more consistent, or more visually telling of why I missed. I had several times where it just seemed like I was raising my sword but not attacking. I'm assuming it's a spacing issue but maybe I just don't know how the controls actually work? If so that's my bad.
Maybe a small indication of how much health you have left?
Loved this one, excellent job!