Quite nice! I see the common 'keep introducing instrument' method being used. Usually I like it, but I feel like boss fights want you to get straight into the action - thus, I kind of missed the thunder, the bang, the fight! Lastly, I think you might repeat your measures a bit too much - 4 minutes for only 2 repeating themes (even with incremental introduction) is a bit much I feel.
But that's enough of me being an ass: besides a bit of a lack of power and a bit too much repetition, I do really like your song (especially those tubas in the second section, damn)! You should be proud of yourself :D