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I have the impish yveltal you are looking for if you still need it :). Ivs are 31/31/31/xx/31/31. Lmk if you still need it :). Has pokerus but no evs or exp.
I have a self-caught 31/31/27/28/30/31 Naive EV-trained Kalos-Born Mewtwo and some Yveltal of various natures I received over the GTS, if that interests you.