Stratovania

Virtually everything does. And in fact that's kind of the problem - it ruins the challenge at endgame. Assuming a high "magic" stat (be it obtained transiently via Magic Plus materia or permanently via consumption of Magic Source items), Knights of the Round can do 129,987 points of damage. Even at low levels it might still do 60,000+ damage. Adding to the insanity, it can also be Mimic-ed and/or W-Summon-ed. Outside of the superbosses, there really isn't anything in the game that can stand up to a dual-summoned, subsequently mimed Knights of the Round. There are only a handful of enemies that can stand up to a single casting.