Kuja would destroy Sephiroth. The whole "you have to choose a certain set of rules" idea is pure crap. This isn't a Zel Melee discussion, therefore we assume they both have acces to everything they could use in their respective games. Kuja just trances, keeps himself in mid-air and destroys Sephiroth with an endless volley of magic. Sephiroth was a wiener, Kuja was the man. The end.