I've searched for this fight in numerous versus forums and never seen it. Who would win if:
1. Xellos V.S. Rezo
2. Xellos with Sword of Light V.S. Rezo with Great Beast Zanafar
Rezo is the most powerful, well balanced human sorcerer in existence. Though lacking in apocolyptic spells such as the Giga Slave and the Ragna Blade, he has shown the he has vast magical powers. He deflects spells arguably more easily than even Xellos, and his copy destroyed half a city just by striking the ground with his magic imbued staff. he easily mind controlled a company of trolls and humans and has mastered both light and black magic. However, though a brilliant researcher in his time, his madness has somewhat blunted his genius. He tends to fight with more brute force than his opponent, Xellos, and is far too overconfident. His body, being human, is also more fragile.
Xellos is the most powerful non-lord monster in existence and is extremely crafty, able to manipulate the actions of both his allies and enemies. He has shown himself capable of slaughtering hundreds of dragons with a swipe of his finger. He is extremely fast and nigh impossible to hit. However, he has been shown to be somewhat unlucky (when his first, seemingly perfect plan to steal Galvayra was foiled when Lina Inverse happened to jump from the roof and land on his face, and when Valgavv's stray fireball hit the ceiling above Fillia, compelling Xellos to abandon his advantage over Valgavv to save her from the collapsing ceiling and eventually losing the fight).