This is amazing! I absolutely love the instrumentation!
At first I wasn't really sure how this fit into the boss battle theme, but as it went on it became so abundantly clear. When I hear this song I especially thing of a 2.5D platformer boss battle kinda vibe, I think this would fit perfectly in that kinda game.
One thing I think this song really displays well is skillful use of dynamics and balance. One thing I personally have trouble with is making my percussion feel like it does enough, while also making it not way too loud. I think this song is a great example of having quiet percussion that does its job perfectly.
Amazing work, and Happy Halloween!