Well...I have one thing to say.
Good but it is missing one thing. In most boss-battle musics it usually starts out with a very simple ominous melody with few instruments. You started that way, but you really need to increase the tempo later on and also add new beats with the drums. However, the one point I want you to know, is add in new instruments...FAST ones! Add the fast instruments within the next 30 seconds and it should be better.
Good job though, and make more!
Thanks!
~water