In order to see this ending, set FF3052 to 22 (to make the first character Onslaught) and FF4080 to 08 (to set the current stage to 9, one past the final boss).
However, it is a bit more difficult to show in the game as normally just playing as Onslaught results in the game glitching out once you face Onslaught as a boss. Much like its predecessor, this one has an unused ending that appears if you beat the game with Onslaught, the final boss.
