Hey, I must have missed this when it was first submitted to the site. Considering that you claim you don't have a lot of experience creating music, I think this is actually really good. There are some instances where there's a bit too much repetition of the same sounds or notes right in a row, and there's one section near the beginning that sounds more like more or less random notes than an actual melody. But still, the choice of instruments and general mood would definitely work well as a boss theme.
I'm not a music expert by any means, though, so this is just my personal listening impression. Feel free to take it with a pinch of salt.