Depending on your max Hp at whatever level you're at.
If its below a certain point say 5000 or so depending on your armor and defense stats.
If you don't use Omnislash Sephiroth attacks you for anywhere between 4000 to 8000 or max damage, if you survive you simply counterattack automatically and he's finished. He only has 1 hp. If you don't survive the attack the game ends, you're annihilated.