>>19265711252+ Atk Technician Mega Scizor Bullet Punch vs. 4 HP / 0 Def Sceptile: 169-201 (59.9 - 71.2%) -- guaranteed 2HKO
Assuming "headshot" means "a Critical Hit" (which is fair, I think), then
252+ Atk Technician Mega Scizor Bullet Punch vs. 4 HP / 0 Def Sceptile on a critical hit: 255-301 (90.4 - 106.7%) -- 43.8% chance to OHKO
Perhaps not.