Oooh, "Il Grande Duele, Part I" by Luis Bacalov. I don't know if it would fit a boss fight - it tells more of a story rather than a fight... but then again, it is called "The Grand Duel", so there's gotta be something to it.

And, uh, Moonlight Sonata? What are you thinking? You don't fight to the Moonlight Sonata. It's slow, quiet, and peaceful for a reason. (Many reasons actually, one being DON'T FIGHT TO THIS SONG. )