Mewtwo, Xerneas, Yveltal and Zygarde have at a minimum, 3 perfect IVs. My Xerneas happens to have four, with a neutral nature. This allows me to calculate its IVs as follows:
HP: 126
Attack/S. Atk: 131
S. Def: 98
If we assume that Def is the same as S. Def and that its BST is 680, we can...