After completing a playthrough of the game you will be asked to save the game. Now when you load up that file you will be able to access the Seraphic Gate. To get here you must go through a teleporter off to the side in Kolton Palace. This can only be accessed on Normal or higher difficulties.
Continue through the Seraphic Gate until you find yourself in the Oradian Dunes. Head as far along the Dunes as you can until you eventually find a teleporter that leads to Vesplume Tower. Once there you will find yourself controlling just Capell. You will now need to defeat Shadow Sigmund. Once you do he will join the party and the achievement will unlock.