Sephiroth has never been killed in his entire life. That sounded bad. Okay! *rewind*

Imagine the hydra of myth. When you chop off its head, two more grow out of it. The only way to permanently kill it was to set a burn to it so that it could not grown back.

We now seek the fire, in analogy, that will take Sephiroth down for good. We seek his weakness, the one thing that he cannot resist no matter how much he puts up against it. An alternate idea would be to get something immortal as well and pit them in never ending combat. The drawbacks are the only person who is immortal is Vincent and her can't put up with a SOLDIER, he's only a Turk. We revert to the weakness plan.

Reply please. Sephy has a weakness.

(or we could just bind him to a chair and make him watch very disturbing things all over again and again)