Scizor-Mega @ Scizorite
Ability: Technician
EVs: 248 HP / 16 Def / 244 SpD
Impish Nature

252 SpA Alakazam-Mega Focus Blast vs. 248 HP / 244 SpD Scizor-Mega: 130-154 (37.9 - 44.8%) -- 5.5% chance to 2HKO after Stealth Rock // Very much in Mega-Scizor's favor
252 Atk Choice Band Kartana Sacred Sword vs. 248 HP / 16+ Def Scizor-Mega: 127-150 (37 - 43.7%) -- guaranteed 3HKO after Stealth Rock

This would not be sufficient
252 Atk Choice Band Kartana Sacred Sword vs. 248 HP / 12+ Def Scizor-Mega: 128-151 (37.3 - 44%) -- 0.4% chance to 2HKO after Stealth Rock