wow can't believe I did that srry for the mixup, here are the correct calcs
0 Atk Skarmory Brave Bird vs. -1 0 HP / 4 Def Mega Pinsir: 276-326 (101.8 - 120.2%) -- guaranteed OHKO
still the guaranteed OHKO and no pinsir with earthquake will stand a chance against skarmory IMO as +2 return still...