Considering that Sephiroth actually has MORE HP at Level 99, one-hit killing is out of the question.
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As for the amount of time it would take, I would recommend using Yuffie with her Conformer (and if you're not then for the love of cheese do so!). The hardest stat to max will be Luck since it's monster is one of the hardest to beat, and Luck is one of the lowest stats (next to Dexterity). If you were determined to max out each character, then I imagine you may end up playing for the better part of 200+ hours.
well are you still up for that? i mean think about it is it really worth it