I want to preface this with the fact that I really like the game, I think the music was fitting, the scenario was compelling, and there were a lot of nice touches within the art direction that really enhanced the experience. Overall, I think outside of initially weird feeling dashing (which I mostly got the hang over) and wall jumping (which I don't think I ever really did, but I did well enough to progress), this was a really well made game.
I say all this beforehand cause I feel like I might be a bit harsh about the boss fight. The difficulty spike is way too big, and for all my attempts I'm not very convinced I was ever even close to winning. There are a lot of individually cool concepts, that alone or maybe with a couple other of the features would probably make a really satisfying fight, but all together kind of felt insurmountable.
Starting off, it does a lot of contact damage and it feels like there aren't a lot of i-frames to keep you from going from full to zero HP super quickly. In theory, the high knockback is what keeps it from happening most times, but in return, I sometimes get shot into the pits at an angle that I can't react fast enough to wall jump out of. And when I do catch myself, sometimes the boss is positioned at a point where I can't land again (or else I'll take damage and get pushed back in), and I can't keep clinging to the walls or climb above it because my stamina is running out.
It's pretty difficult to find openings to attack as well. Since your attack range is kinda tiny, you have to get pretty close to deal any damage, but the boss's kick attack comes out pretty fast to punish you (also initially, the black fog surrounding it made it hard to tell that it was a follow up attack, and for my first few attempts I thought I was just accidently running into it after it finished the charge). I thought that it wasn't so bad since I could at least do more damage with the new light attack, but since it's a limited resource I couldn't use it all the time, I wanted to wait until I was sure it would hit since missing still loses me the charge, and the enemies only dropped enough energy to use it one more time.
Finally, the second phase is completely immune to the normal attack, so you have to use the special attack. Which I would have used on the first half of the fight, so I'm stuck running cycles around a faster and stronger version of the boss I was already struggling with, waiting to kill the blue spirits to get a chance to take another swipe at the boss. So after a couple of times trying to do that, I started not using the light attack on the first version of the boss so that I could go all out once the boss transformed, but that means the first phase takes twice as long now, and I'm getting pings from all the extra orbs I'm leaving around since I'm full on energy.
I did see your other comment about not wanting to change anything close to the deadline just in case bugs showed up, so I assume you might have already been thinking of dialing it back a bit, but hopefully this at least helps illustrate what some of the players (or at least this player) struggled with during the fight. I hope I didn't come off too harshly, because I did really like the game. I really like metroidvanias, so I'll be looking forward to more in the future.