Not even the Warriors of Darkness could stop the Warrior of Light. Not even Ascians could stop the Warrior of Light. Not even the Dread Bahamut could stop the Warrior(s) of Light. What in the universe is going to give the WoL pause?
Zodiark?
EDIT: The closest I've seen the WoL come to defeat was at the end of 2.55, by a political giant. It took a political giant to nearly bring the WoL down. War is awfully political, especially when it's not Ishgard that's on the brink, but Gridania, Limsa Lominsa, and Ul'dah. I think we need lots and lots more politics, THAT'll make the fight more difficult.
Politics.
EDIT EDIT: Getting back on track of this thread, I think Y'shtola will bite the dust. I don't know about anyone else, that remains yet to be seen.