How do I beat Nemesis?

[Yuriev's ghost]

Wakka + Max Strength (level it up by killing Juggernaut over and over again) + Celestial Weapon + prefect Attack Reals = 99999*12 damage each time you use it. Even 10 million HP doesn't take long to chip off when you can cause that kind of damage (every few turns, if both your other characters have entrust). Just use an aeon to take Armageddon. He casts it between the 9th-11th turn or something, so summon one around then and just be sure to keep them alive to the point he uses it.