Not the most experienced battler but I guess Firium-Z Rotom-H comes to mind. Though you could argue other more defensive Rotom-H sets are a harder pure counter with leftovers since it allows more hits to be tanked.
Rotom-Heat @ Firium-Z
Ability: Levitate
Level: 50
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Overheat
- Volt Switch
- Hidden Power [Ice]
- Thunderbolt
Outspeeds any variant of Mega-Scizor at 151 vs 139 speed at max Speed EV positive nature
Overheat OHKOs, assuming it hits, any variant of Mega-Scizor or Firium-Z is an alternative for accuracy/no special attack drop.
252 SpA Rotom-Heat Overheat vs. 252 HP / 252 SpD Scizor-Mega: 304-364 (171.7 - 205.6%) -- guaranteed OHKO
Able to also keep momentum with another move predicting the switch.
Calcs for Bullet Punch/U-turn:
+4 252+ Atk Technician Scizor-Mega Bullet Punch vs. 0 HP / 0 Def Rotom-Heat: 44-52 (35.2 - 41.6%) -- 90.8% chance to 3HKO after Leftovers recovery
+4 252+ Atk Scizor-Mega U-turn vs. 0 HP / 0 Def Rotom-Heat: 103-122 (82.4 - 97.6%) -- guaranteed 2HKO after Leftovers recovery
Can always switch in on BP and force Scizor out, tanks even +4 U-turn unless there are rocks on the field.
Rotom-Heat @ Firium-Z
Ability: Levitate
Level: 50
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Overheat
- Volt Switch
- Hidden Power [Ice]
- Thunderbolt
Outspeeds any variant of Mega-Scizor at 151 vs 139 speed at max Speed EV positive nature
Overheat OHKOs, assuming it hits, any variant of Mega-Scizor or Firium-Z is an alternative for accuracy/no special attack drop.
252 SpA Rotom-Heat Overheat vs. 252 HP / 252 SpD Scizor-Mega: 304-364 (171.7 - 205.6%) -- guaranteed OHKO
Able to also keep momentum with another move predicting the switch.
Calcs for Bullet Punch/U-turn:
+4 252+ Atk Technician Scizor-Mega Bullet Punch vs. 0 HP / 0 Def Rotom-Heat: 44-52 (35.2 - 41.6%) -- 90.8% chance to 3HKO after Leftovers recovery
+4 252+ Atk Scizor-Mega U-turn vs. 0 HP / 0 Def Rotom-Heat: 103-122 (82.4 - 97.6%) -- guaranteed 2HKO after Leftovers recovery
Can always switch in on BP and force Scizor out, tanks even +4 U-turn unless there are rocks on the field.