Alright! As promised. Much of this review is gonna be tentative, because I'm not familiar enough with the engine to tell what's a mechanical limitation and what's a design choice. All crit is meant to be constructive- I did really like this one, I'm just digging in.
The bosses were fantastic- the callbacks on the third boss really made it for me, especially because the adaptations removed a lot of weaknesses of the previous versions- hiding behind the lower piece of the ping pong mechanic from boss 1 was no longer a good way to avoid damage, etc. The animations were crisp, clean, and choreographed the attacks well (look at that eye tracking on the second boss! wow!), at least after I'd seen each attack a time or two.
The animations and movement controls felt good, and just floaty enough to sell the falling part while maintaining tight controls. I will take some degree of issue with the walls however- at the speed of movement, the relative (normally good) monotony, and the bright colors required by the palette, it quickly became slightly dizzying and broke up my ability to focus on the foreground action or register player movements- this was worse with some boss palettes than others, though it remained an annoyance throughout.
I'm less a music person, so the most feedback I could give for the music is that it was great, and I very much appreciated the callbacks. If a volume / sound slider is possible within the engine, I would've liked that though, to turn the effects volume up a lil bit.
Balance prolly needed a little bit of work- I did two plays, once with the bones + arm and once with the eyes, and my second go of it was much easier than the first- as sometimes one of the bones would get lodged somewhere complicated and I wouldn't be able to shoot, nor reach the third boss where it was hiding under the... water? acid? for a while. I also wouldn't call the bosses easy per say- so much as legitimately well balanced for the size and scale of the game, though I will admit I spent a minute or two shooting the tutorial skeleton in the hopes he'd turn out to be a secret bonus boss or some nonsense.
I wasn't entirely sold on the theme fit at first, but between using bits of the former bosses that also recreate their mechanics and the nature of the third boss reusing the same, I wound up sold on it by the end. Good theme use on a very classic boss rush.
I do wish you'd done a bit more with the fact the player is falling more than just on the stylistic end, but as a gamejam game this is a super cool entry and definitely one of my favorites thus far! Really did like it, and hope it does well in the actual contest!