I've done this. All you need to do is go fight Magimaster if the Advance version, and have Locke Steal from him. Even if it keeps saying "No Item to Steal" you may have to steal anywhere from 50-100 times to get the Chrono Cross. Using it you can traverse to an alternate dimension and fight as an entirely new group, consising of Kefka, Ghestal, Leo, ???, Banon, the Major of Thamasa, Hidon, and Arieth to fight against the evil empire lead by Emperor Locke and his trusted General Sabin, who later betrays him to create the empire of death and destroy the face of the world.