Changing Battle BGM for a boss battle.

(XP) ^How do i do it?

Is this XP? You use the event command, Change Battle BGM

You put that in the Monster Group tabs in the Database where you insert event commands in the lower right hand corner of the Database window. Be sure to set it so that it Auto-Starts on turn 0 and that (probably) you use the "Erase Event" right after. Also, remember that in the event that you make the boss start a battle that after you put the "Process Battle" or whatever that you change the BGM back to normal. Hope I helped

For that you need to use Memorize BGM on the map where the event goes to the battle, and recall memorized bgm once the battle's done.