Actually, I would have formatted the battle like this:
Sephiroth (normal self): Acting as Jenova's guardian as she prepares Meteor. (Well, maybe I'm messing things around, but suppose Sephiroth gives temparory control of Meteor to good ol' Mumsy while he fights AVALANCHE.)
Jenova SYTHESIS: True, a more endurant version of the original Jenova SYNTHESIS would suffice as the 2nd Final Battle. (Frankly, she was too easy to beat, even without the Knights.)
Jenova the One-Winged Angel: Sorta like Safer Sephy, but It's Jenova's final form. And let's not exclude the Super Nova/Reflect/Wall. Hehehe.
Normal Sephiroth with 1 HP - Well, exactly like the final FINAL battle with Sephy and Cloud, except you FORCEFULLY use Omnislash instead of defending. Give the viewers a show.

That's my theory. If people disagree, then lemme see your suggestions.

Peace, MasterX