This is some pretty interesting music, even though it doesn't sound energetic enough to be much of a "battle theme" to me. The Undertale sound effects were a fun touch (psst, by the way, because of those you need to mark that your track has 3rd party samples in it). My favorite part is around 01:15 - it has similar vibes and even similar instruments to Terraria's nighttime music. It sounds like the synth doing the Megalovania-esque thing comes in a couple beats early at around 1:44, which means it doesn't always line up with the other stuff. But otherwise, really nice work, and I hope you return to this idea later!

Shady Cicada has a few videos on making video game battle/boss theme music, maybe those might be helpful?