The reason Drain/Demi type spells do so much damage to Altima is because they're percentage based. When you're fighting ordinary enemies that other units can kill in one or two hits, an attack that deals 1/4 (or 1/3, I can't remember) the max HP of its target is pretty meh, but when you're fighting a final boss with 2000ish HP, they're pretty spiffy.
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Mistake you're making - overlooking the fact that we might not want to be saved.