Defeat Lavos at the end of the game.

Defeat Lavos while Chrono is dead.

DefeatLavos in the Black Omen OR right after Marle joins your party.

Defeat Lavos as soon as you return from 600 AD.

Defeat Lavos as soon as you get to the End of Time.

Defeat Lavos before you get the Hero Medal, but after you hear Tata is the hero.

Defeat Lavos after getting the Hero Medal. But rememeber to do it before the Masamune is fixed.

Defeat Lavos after you get the gate key back in 65,000,000 BC.