I like your take on the theme, how you have to stop the others being stronger together works really well both narratively and mechanically.
My main critique would be that the levels all the way up to the boss are a real breeze (too easy), and as such didn't quite prepare me mechanically fo rthe difficulty of the boss fight.
Similarly i do love the simplicity of your control scheme, but it definitely felt a bit spammy (at least for me), the best tactic seemed to just run into the enemy and tank the damage... haha.
It also would have been nice to see the arenas change slightly each time to fit the theme of the new enemies we were encountering as we went deeper into the dungeon.
That said, the sprites are really nice and hold a lot of character, the mechanic is great and the whole setup makes it all feel really charming.
Great job!