Sephiroth was one of the last things I did. By the time I went after him I was already in the high 60's. From what I've noticed, level doesn't matter very much when fighting Sephiroth. Since HP isn't based on your level, that's not a concern. It's all knowing his attack patterns anyway.
True! All you need to beat seph is know-how. His attacks are easy to dodge/counter if you know how. i beat him at lv 55 but i think i would have no problem beating him at lv. 30.