I like the ideas, but I think the combat misses some pretty crucial aspects of Undertale's gameplay and also bullet-hell's more broadly: there's supposed to be some way to see attacks before they can hit and always some way to avoid them. The first attack, with the bird flying straight at the player doesn't seem to have any way to avoid it quickly enough, even from clear across the arena, and after that most of the attacks have some aspect that can hit the player instantly. Separately, I think the two revolving bell shapes were unfair as well, due to hitting in spots that didn't match the visuals.
One other thing, is Sandalphon a reference to Shadowverse, and if so, didn't of them actually use an evo deck?