This takes advantage of Edward's auto-hide property when he is in critical status (low HP). First, berserk him, then take him down to critical (tornado works well). He should hide in his next available turn. However, he retains his berserk status, and in his next available turn, he'll reappear and attack again. He will still be berserked, but he'll also still be considered "hidden." That means that he'll constantly attack and never be able to be targetted, even by spells that should hit the entire party, as long as one other person is still alive (he can't hide if he is the only person left alive). On the negative side, it also means you can't heal him, even with party healing spells.