I normally just use the normal music in the game but the Ys Series and Legend of Heores series OST are just battle and Boss battle worthy of pure epic proportions.

such as

EXceed
http://www.youtube.com/watch?v=27YXozjUnlA

Seeking the Vanished Mask
http://www.youtube.com/watch?v=KfTrdb_taP0

and
Inevitable Struggle (Evolution remix version)
http://www.youtube.com/watch?v=nre26zOOCOw