@rizap, you are semi-incorrect, I use Stakataka in Showdown! and have this spread: 252Atk, 252HP, 4SpD, IV's: 14Def, 0spe, atk stat is raised by beast boost, he is using level 50, While you use lv 100, which needs 1 less def IV
That being said...
@ThinkGaster10, specify that the Def IV can be 15 to boost atk at lv 50, but needs to be 14 to boost atk at level 100.
Thanks :P