I don't have experience with bullet hells, including undertale, so I'm not sure how it stacks up to similar games. But, this game was intriguing enough that I wanted to finish it. My thoughts are below:
-The final boss is hard, but also rewarding.
-The characters and flavor are appealing.
-Enemy positioning is clever, and there's a lot of potential to make some crazy maps with this system.
-The final boss being Hollace is predictable.
-It's amazing that you were able to make bullet hell mechanics work in this engine.
Keep on innovatin'.