Well, not giving away spoilers:

Kuja is simply a powerful mage. His build is light and he never employs any physical attack. Simply magic. He too has a devious mind, and manipulates a continental war and chaos, through the greed of another. He has a mind for deceit and mamipulation, same as Kefka.

Kuja, at the peak of his power, was able to annihilate a planet. He also took out your entire party in one attack. He is a cocky person, but he is a no bullshit fighter. He'll kill you, laugh while he's doing it, and then just walk away, after making a eloquent quote of some kind.

I'm not sure who'd win, but I know that it be a spectacular sight to see.