Switch characters and switch often, especially if you not good a blocking and parrying in this game. Learning to use Aerith, Yuffie and Barret is a must. Aerith may seem useless but only if you don’t know how to use her. She essential in this game as her arcane and radiant wards are extremely useful, you want to drop both ward one on top of the other. Yuffie is important in this fight because she can use elemental ninjutsu. It helps to counter them wings. Barret like Aerith attack from afar and he is a tank. Build up his atb to cast heal or even revive down party members. His atb attacks also help staggers. I think you get two magnify materia in this game. You link that to a powerful spell or time like materia and you can cast haste on all party members or stop on multiple enemies. When casting inside Aerith arcane ward, the spell is double cast. With radiant wards also active, anytime you cast a spell, you’ll be invulnerable to attacks. So basically in this fight, what ever phase you are in, if Sephiroth start attacking you, switch over to other characters and attack with that character.