You can't actually "beat" Sephiroth, you just survive long enough to go to a cut scene. I recommend using a phenix down before the battle, equiping Costly Punch, Jump, and Cure or Curaga. The First...
after you fight him as the final boss he collapses and sees the goddess he never shuts up about and she lets him go and Zack resceus him from dying so genesis is lying on the floor besides Zack and...