The way is to reduce his HP to zero before entering a battle. Since he is invulnerable and cannot be attacked,(Trust me, go ahead and attack him with everything you've got)you will have to use some kind of cheat code to reduce his hp to 0 before the battle begins. The game will still think his alive when you check the status screen(his HP is in yellow not in red). Anyway, just enter the battle and you'll see seph just standing there(he doesn't have a dead pose) so revive him with a phoenix down or something and you will then gain full control over him. Unfortunately, Seph will now be able to be injured(you turned him mortal!), and you will have to repeat this process before every battle. A better way is that once you can control him, just kill him before the end of each battle.