I actually managed to beat all the bosses no hit (if there is green inside the difficulty color circle, that means you beat it no hit), on all 3 difficulties! I raged ALOT and went insane ALOT, but eventually after so much torture and trial and error, I finally managed to do it! I might be the first person ever to get all difficulty no hit completion checks in the game! The hardest ones (to no hit) were def Rainbow Dash on any difficulty (She gave me a HARD TIME to no hit in easy mode for some reason, after having no hitted her in the other harder difficulties), and Trixie on Hard Difficulty!
The only sad thing is that, you only get supreme ultimate bragging rights for getting all no hit checks for all difficulties. One of the biggest factors leading my determination to get all no hit completion checks, was that there might just be a secret or a secret boss fight waiting, if you get all no hit checks, but its understandable since this was a game you worked on long ago in the past. Despite not having a secret boss fight or any secret really for getting all no hit checks for all difficulties of all the 4 bosses, this is still a really fun and challenging as heck game that holds up!