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Maybe for Yveltal, but I don't think Zygarde can really do anything to Xern anyways/Zygarde doesn't want to eat Moonblast (and can't stop Xern from setting up with Geomancy).
Like I said, the ability is godly good for ubers but the Pokemon that has it is counterproductive considering that it is weak to fairy anyway. Zygarde is garb anyway so its not like its a big deal or anything.
Good to know still, thanks!