Yunalesca was a cinch once you figured out the trick.
*spoilers below*
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*spoiler warning*
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When she turns you into a zombie, KEEP THE STATUS. That way her move that attacks with 80% chance to cast Death or something (while dealing rather piddly hp damage) doesn't score, since you're already dead. You can also cast reflect on your characters to protect them from Yunalesca's healing spells, which will reflect back to her of course, but it's better than your team dying. Having nullified her best souces of damage, smash her with your own healing spells, or Holy if Yuna has it.
It's also important to know that Reflect only works in one direction, one time. If both your party and her have reflect cast upon each other, cast spells upon yourselves to make them hit her, and vice versa.
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I believe I am cousin.