when i did it, i just did what i normally did, which was
1st move slightly backwards
2nd dodge attacks
3rd when he uses his special move just defend before it goes into that cut scene bit
showin u his special
4th give it ur all just go straight at him ( WARNING it depends what lv u are if ur a high
level it shouldnt take u long but if ur lv 20-25 train until ur atleast lv30
5th EQUIP VITAL SLASH AS IT DOES HIGH DAMAGE IF UR LV 30-100 IT SHOULD DO FROM 5000-9999 I
AM LV 55 ON MY 4THGAME OF IT AND IT DOES 9999 EVERY TIME

hope this helped

Sephiroth blows you away ,
You Press SQUARE and UP u'll
roll forward and hit him he'll
go backward.. (VICE VERSA :D)

OR PRESS TRIANGLE U CAN JUST BLOCK IT..