The problem is the game checks for monster AI first, before checking to see if the party is wiped out.
One is to add a condition to the "Kefka death" animation of his script that someone has to be alive in the party
If not done properly, Kefka will just instantly die then (without his fancy fadeout).
and the other is to make the animation void party annihilation.
I have another idea. Upon death, set his HP to full, and then check to see if the party is alive. If so, fancy death. That way, he doesn't insta-die, but then he'll still go through the animation if you win. Like this:
FC 12 00 00 ; If following monster is/are dead:
F5 00 00 0F ; Monsters #1, #2, #3, #4 , if hidden/dead, brought in with their HP restored, suddently
FC 13 00 01 ; If 1 ally(ies) or more remain
FA 0D 01 00 ; Background gets darker, sounds like boss dying
F5 11 01 FF ; Monsters #1, #2, #3, #4, #5, #6 are killed, disintegrates, background may corrupt, screen goes black (final Kefka's death)
FE ; End If and reset targeting