Okay I finally got to play it! Thanks!
The game was fun, and the art, sound, and music were all great and fitting! I love the concept of the game, although as has been stated already, the theme on collection is barely used.
I had several issues with the game as I played. The first boss was actually the most difficult for me because he liked to use a charging dash attack, which he gave no sign of preparing before suddenly launching into it. Unlike his (much rarer, I didn't even see it in my first playthrough) crouching projectile attack, which I knew was coming after the first time due to him crouching. There was no way to predict the dash, from what I could tell, except to stay far away from him as much as possible. This made the battle take longer, and it wasn't very fun, because it was mainly just: rush in with proper timing, attack once, retreat quickly and wait, then repeat.
The second boss was much more interesting and fun to fight. They telegraphed their moves well, meaning I could predict what would happen and adjust my playing accordingly. However, there was one issue that made them too easy to defeat. Sometimes, they would not do any attack at all, and worse, they would teleport to a spot next to one of the pillars. This let me rapidly attack them over and over. On my second playthrough, I took advantage of this and I didn't even need to worry about their explosion attack, because I knew I'd get my health back at the end of the battle, so I just kept spamming my attack until they died. This let me beat them very quickly. Also, the dash was practically useless because it was too difficult to use quickly and didn't seem to give me much benefit when I pulled it off, so I basically ignored it.
The flying orb attack that he had was the most challenging attack to deal with, and I think that would have been better used as a permanent addition to the battlefield after the boss was at half health. This would have prevented me from just standing there and spamming attack, I'd be forced to move around. Although if this was done, the dash would need to be improved as well.
The third boss was... ironically, the easiest by far. I just had to stand there and spam the attack button as fast as possible once he got a little close to me. I'd occasionally have to jump to try to avoid the skull things (I stayed on the top-left platform, so I only ever had to deal with one at a time), but then I could go right back to spamming. In fact, avoiding the skulls was pretty difficult due to the timing needed, so it was more efficient to just keep attacking. I beat him really easily and quickly.
I'd suggest giving him more unpredictable attacks using his tentacles, or maybe a laser coming from his eye that shows he's charging it before unleashing it, making you want to leave the safe platform and risk dealing with the skulls. On the note of the skulls, I didn't realize at first that you could kill them, so I legitimately thought it was part of the challenge that he keeps summoning more. This ties back to a problem mentioned already, which is the lack of health bars (or any visual indication of damage) for the enemies. The 1st boss doesn't show ANY signs of taking damage until you kill him. The second boss at least flickers when you hit. But it's only the third boss who noticeably has a reaction to being hurt (although even in his case, you can't tell how wounded he is). The skulls show no sign of being hurt by your attacks, making it seem like it's useless. So more clues as to how hurt the enemy is would help a lot.
Finally, I went back and played the game a third time, and found out that I can beat the first boss just by standing there and spamming my attack without ever dodging once. I think the player might have too much health, haha. Oh and by the way, there's a typo at the end. Instead of "THANK'S FOR PLAYING!!" it should say "THANKS FOR PLAYING!!".
Regardless, it was fun playing the game and it's impressive that you did this in such a short time. Great job! I hope my feedback helps.