Someone has never played FFVII. Or X. Or Wild Arms 2. Or Lunar 2. Or a lot other RPGs where there's an extra battle after the actual final boss to wrap up the story, generally involving some special ability awakened in the player party that can destroy the boss. In other words: Ultima Weapon is the final boss, Lahabrea is a scripted, nearly impossible to lose battle to wrap up the story. It's the equivalent of watching a cutscene where your group banishes evil once and for all, except you play it out.

Run Praetorium again and pay attention to your buffs once you reach Lahabrea.