Legitly, if you want the perfect Nature(using Syncronize ability) and either 30 or 31 IVs for 5, it's 1 in 10,240 chance. I probably spent a total of 3 days soft resetting for Yveltal for months.
Thanks to Powersaves, I've just caught my Mewtwo, Zygarde, Articuno, Zapdos, and Moltres with their...