only heal two and try to use remedy to heal them, save Yuna for the end of the battle, try equipping things that will keep you immune to a status effect. this is the basic advice.
Now if you want to defeat her there are several ways, first is heal with Yuna status effects and try to keep her safe for damage (same as the tactic that Member XIV mentioned), this is the basic one for which is reccomended to have ethers and remedies, the second way is to take advantage in being a zombie, white magic does hurt you but certian black heals you so you can have Tidus for example as that attack force and with Lulu and Yuna heal him (whit for healing when he is not a zombie and black when he is) and there is the third way, cast reflect on the party and start hitting her, but for this one you need a lot of potions.