I never found Demon Breath 2 to be a problem because, due to that spell, I've always attacked him with only one unit at a time. And because the enemies in the battle are so spread out across the huge battlefield, I've never had any trouble keeping everybody alive to fight Zeon with in that fashion. When he can only hit one character at a time, you have more than enough firepower to gradually take him down.
Regarding Karna, I basically never use her, and even if I do it's not like I ever get to a high enough level with my healers to learn spells like that. Pretty much the only Creed characters I ever choose are Tyrin, and Eric, and while I do sometimes go ahead and recruit the others later I'm generally not really interested in stopping to grind in a game where grinding isn't mandatory.