I love the buildup you have in this song with brass instruments, especially combined with the piano climbing up the key. This really has strong horror vibes to it, which is great.
One thing I especially appreciate about this song is the moments of silence that separate parts because it really helps give each part more weight to it.
Beyond that, my main concern with this one would be that I'm not getting very strong "boss battle" vibes with it. If I were to make it sound more that way, I would definitely try to make it more intense by focusing more on the percussion and maybe speeding it up a bit.
This sounds great, keep it up!