You must take note that some bosses "need to be killed" more than once...Quote:
when it comes to boss, wait till it dead, or read the dialogue, when u sure it's already dead, deactivate, and press X., :pimp:
Ex : Victor, Thanatos
And for those bosses, press Deactivate (F3) only after his last transformation is dead or u'll be kicked out.
And I noted from my experience, for some dungeons don't press X (skip dialogue) too quickly right after entering boss room. Or u'll get un-killable boss without any HP bar, because the boss was re-raised by the game's event itself... (LOL)
I observed it'll happen only on Manna Valley (3rd Xenia), and Temple of Cuatal (2nd Xenia).