That really really stupid boss is the beginning of the final bosses.

Well, this is what I did to beat all the final bosses: Yuna and Rikku were dark knights, Paine was an Alchemist. Darkness everyturn, mix/stash mega-potion every turn. It wasn't that hard, none of my characters came close to dieing. I might of been overleveled though (Paine was 84, Rikku was 82 and Yuna was 81).