(SPOILER)Actually you don't have to go through the Mysidian Tower again, it's a completely optional quest and it's only the top floor of the tower with no random encounters. Just Ultima Weapon. You also get to fight alternate versions of Tiamat, Beelzebub, Asteroth and the Iron Giant named Yamata no Orochi, Beelzebub Soul, Lucifer and the Steel Giant, respectively.