Hey, thanks for the feedback! I'd like to clarify some of your points so that I could maybe clear up some confusion or fix the bugs you've found!
1) You won and the game broke: What broke exactly? Did the game crash?
2) If you're playing on "Hard", then yes the boss can "shoot infinitely" as his attack timer is reduced. However despite this being a very difficult gameplay mechanic, it does not make the game impossible to beat on that difficulty. I have a gameplay video beating the Hard mode here: https://www.youtube.com/watch?v=SfMLbgeaKQE.
3) I playtested a 4-5 times to recreate the issue where he idles below 10%, but was unable to do so. Is there anything else that you were doing during this time that may have been a cause of the issue? Are you able to recreate it after playing again?
There is a known bug that if you play, win, then play again - you will be unable to win. I'm sure it's due to a static variable that isn't reset between scene transition. Is this one of the bugs you were referencing?