In the last battles (Trance Kuja and Necron), it isn't necessary to have a white mage (Eiko or Dagger), Vivi can heal your party.
To do this, first you need Zidane, Vivi, Amarant and Steiner. You need to equip Zidane, Vivi and Amarant with armor that absorbs shadow damage. You can't equip Steiner, because there isn't any armor that absorbs shadow damage. Then, have Amarant use Aura on Steiner, aura casts auto-life and regen on one target, and then have Vivi use doomsday. It will heal Zidane,Vivi, and Amarant by 9999, do about 5000-7000 damage to the enemy, and kill Steiner. But, Aura kicks in and Revives Steiner.