Wandering Flame
Restricted
The whole thing about SHA depleting health and mana to 1 is its gameplay description, as much as a sword swipe taking (insert amount of HP here) away from ones life bar.
I'm positive the canon description of SHA is taking life away (it happens instantly, but a few seconds after the spell is casted).
That probably would work here if Hayabusa was fatigued like hell later on in the battle. He has superspeed + teleportation to counter with if he is not fatigued.
Though nothing stops Sephiroth from simply snapping his neck the same way he collapsed Shinra Tower.
There I go again talking about FF7 Sephiroth.
One of FF7 Sephiroth's many abilities (telekinesis) has not been demonstrated by KH Sephiroth, so that's not an option of winning in this case.
If KH Sephiroth uses SHA while either raining meteors or casting pillars of flame (that surround him) he would win (that's if he can do both at the same time which I believe he can).
That's nothing more than my honest opinion.