This may be a good point to utilize the different tiers of Kefka's final boss...like, they could all be fighting the different tiers simultaneously, and you could split the party up that way if you wish. Kefka's power is drained as each of the different bosses are defeated. Kefka then does some move that flattens the whole party, but Terra, Celes, and whoever else you want gets up for the final battle. Insert "life...dreams...hopes..." speech. It's not until this point that Terra turns esper. Then, if you must, Cyan can run his sword through Kefka as everyone else focuses their powers to Terra or something, who casts the final utlima that destroys him. Or like, everyone could take turns using their "limit breaks" (they sort of had them in FFVI, not that I'd ever let hp get low enough to use them). While I would personally want Terra to have the final blow, I'm not all that concerned with her having a 1-on-1 with Kefka; we had that in Dissidia, and this is not Final Fantasy VII with Cloud and Sephiroth. Almost everyone has some sort of personal score to settle with Kefka, and its all about regaining hope in their lives despite their individual tragedies (glares at Cloud. Actually Cloud's emo-ness doesn't bother me that much. He's just an easy target).
I'm also not too big on Cyan giving the killing blow...but this is your show, so do what you want.
There could also be some sort of special thing that Terra (or other party members) channels the powers of all the espers or something, to make Kefka's defeat in live action more feasible.