Scizor can survive for a several hits from Weavile, beeing a Steel-type and having great defenses.
Scizor doesn't have chance with Speed but Bullet Punch is super-effective and priority move.
Weavile has a base of 65 Defense.
252+ Atk Technician Scizor Bullet Punch vs. 0 HP / 0 Def Weavile: 306-362 (108.8 - 128.8%) -- guaranteed OHKO.
Of course that using Swords Dance first it's better. But while using Mega form:
252+ Atk Technician Mega Scizor Bullet Punch vs. 0 HP / 0 Def Weavile: 338-402 (120.2 - 143%) -- guaranteed OHKO.
Steel and Fire types says all! With great defenses and great Special Attack, it can survive any hit from Weavile. Flamethower on Heatran is deadly!
252+ SpA Heatran Flamethrower vs. 0 HP / 0 SpD Weavile: 372-438 (132.3 - 155.8%) -- guaranteed OHKO.
Any Pokémon that can Wall a Dark and Ice type moves, and have a great STAB super-effective will be able to OHKO him.
Hope this helps.