Hmmm... If we put Seph at about 7-8, Cloud, Leon, Yuffie and Tifa would be about... 11

Think of Sin Harvest - multiply it by four - remove the time limit to cancel it - you get the 49th level of Hades Paradox Cup...
EDIT: I CANNOT believe I'll dispute myself... The previous was written while I was on level 41 of the HP cup, and I have only heard about Cloud, Leon, Yuffie and Tifa fight from the walkthroughs that said it was impossible... I completed the cup. Personally, battles 41-47 were A LOT TOUGHER than the Famous Four Fight. My stats (If you thought I was upgraded like Robocop):
Level: 88
Strength: 69
Magic: 59
Defense: 70
Abilities (just the crucial ones): Glide, Air Combo Plus X 2, Combo Plus X 2, All of the combo and finisher moves upgrade (Aerial Finish 'n' stuff), MP Rage, Haste, Hastega - from Ultima... And so on.
Play level: Standard (I now you're gonna be pointing at me because of this, but many people play on this level, and I'm starting Proud soon, since I've beaten the game, and I don't think there will be much difference)
They were amazingly easy, and they fell down in this order: Tifa, Leon, Yuffie, Cloud. Just do this: Jump, do an aerial combo on Tifa, Glide, Thundaga or two, heal if you got hit by their combos (or caught in a sandwich between all four), and keep on doing this (change from Tifa to other characters or start with another, but don't start with Leon, he's got a mean range gunblade attack) 'till you have won. I didn't even use Trinity. Well, I've used it once, on Cloud, but as a finisher, and it takes their health down like crazy (at least Cloud's)... Kill Leon off with normal, ground combos, and Yuffie and Cloud the same way you did Tifa. Tifa's the easiest one to kill, mainly because: Leon just stands in the center, waving his sword, Yuffie teleports around, Cloud flies... Tifa runs. If you glide, you can easily kill her. When she attacks you with Final Heaven, just JUMP. Whenever they attack you, JUMP... It's pretty monotonous...
Like I said, battles 41-47 were way harder for me. Morning Stars, Living Bones, grrrrrr!!!!!!!! Don't worry about Hades, Herc will be there to help you.
Hope I've been a good enough help for you to beat the Famous Four Fight. Cheers, and enjoy the game the way I did.