Okay, to answer your question, Kefka would leave Sephiroth buried under Figaro Castle and laugh about it.
Kefka not only has a staff he can attack with, but high level magic abilities as well. And if that isn't enough, he has the power of tons of espers. Plus, what does Sephiroth have? If need be, Kefka can call upon an entire army of soldiers with varying amounts of strength. He has airships and weapons and factories at his beck and call.
And when Sephiroth is being swallowed by the sands of Figaro, Kefka will let out his infamous and ever feared laugh. And should Sephiroth survive, Kefka will just have to chase that son of a submariner down.









