I got multiple but I will go by character. I may include beforehand fight as I do not think of Boss Fight unless I am sure their death is near or I am ready to let them go. Some may not have a true boss fight as I have not really found a good one for them yet, even after all these years.
Crane Baxa: Furfilling His Destiny
I never let him lose control, when it finally happens, this be the song that should play.
Jegy Sesara: My Lovers Words
Jegy has a complicated history with women and it affected him. Each one he became very romantically involved with, he gave a promise. He honors each of those and is the reason why he can never be truly evil, for he honors every promise he made to them, in their memory. I may play this before the end, one last reflection, letting them know he will soon be with them again. Just one more fight.
Haon Hafey: The Duel
It started with a Western, it should end like one. Haon Hafey is always Paladin to me, it be fitting if his final fight ended in a duel.
Callum Saville: Boss Theme
What else? Loved it since I first heard it and it fits.
Dalos Cameron: Boss Theme
All he has known is bloodshed. Memories lost, he must fight to find purpose.