Not a problem. Oh, and the very very last resort: Level like hell. It never fails. XD You might have to in order to get Holy for Yuna, as it is at the very end of her sphere grid. :x
It might help doing that once you get into the third form and onward (or are about to), as the aeons are immune to Megadeath -- or I think they are, I've forgotten. Regardless, it wouldn't be Game Over if they die, and she doesn't cast it every ten seconds. It could be an alternative if you just can't find Death Ward armor anywhere. I think you should, though, if you dig around a bit.
I know! I can't believe I forgot about it when I fought her, as that was the largest problem I ever had with her. -_-; She's not that hard to damage if you're leveled high enough.