In a normal fight Exdeath would win. That guy already knew Meteor, Holy and Flare and is able to use them in 1 turn => Kefka is pwned.

God Kefka vs Tree Exdeath is Exdeath yet again, Exdeath would just suck Kefka and his tower + statues up in the Void. The end.

God Kefka vs Neo Exdeath, well, it's hard. Neo Exdeath seems to be the Void itself, so it can be assumed that Neo can suck things up in the void as well. Besides, God Kefka's attacks aren't really a threat to Neo, except maybe Fallen One. Well, it depends. If Kefka can use Fallen One and soem all-hitting attack (Goner) before Neo can suck him up in the Void, Kefka wins. If not, Neo sucks Kefka up in the Void and there goes the Godly clown.

Story-wise, Exdeath wins for me.