Sephiroth's "Wall" blocks against both Magic and Physical attacks, whereas Kuja is confined to just "Reflect" which repels Magic. With this on, no Curaga should be used, obviously, because it would simply help Sephiroth.

Kuja needs to maintain distance between himself and Sephiroth. The Massamune could be used against him, and since he has no weapon of his own, a close-quaters battle only makes the battle that much tougher for him.

The Funhouse might crumble with the magic attacks flying around, demolishing walls and pillars. So, both would have to avoid falling objects.

It's almost dead even magic-wise. Both their attacks are equally destructive and devastating.

Kuja could use Ultima, or even Flare Star, both large-range attacks that would attack all the areas around Sephiroth. However, Sephiroth's Supernova and Stigma attack a large area as well.

It's almost dead even.