I agree that Clefable's probably the best option right now, but here's another option for people who want more choice.
Raikou @ Assault Vest
EVs: 252 SpA / 32 SpD / 224 Spe
Timid Nature
- Thunderbolt
- Volt Switch
- Hidden Power Ice
- Extrasensory
Mega-Beedrill hates flying-types like Mega-Pinsir and Talonflame, who could ruin its sweep with a priority flying-type attack. Our team is also very weak to Keldeo, who can hit Lando-T with Scald and Porygon with Secret Sword. AV Raikou, while not being able to counter, can act as a shaky check by switching in to a flying-type move/special attack and forcing Mega-Pinsir or Keldeo out. Raikou can also take down Skarmory, who would otherwise completely wall Mega-Pinsir. Finally, Raikou can use Volt Switch, which helps to maintain momentum and get Mega-Beedrill safely in.