Mega Banette @ Banettite
Ability: Frisk / Prankster
EVs: 252 HP, 252 sp. def, 4 def
- Pain Split / Taunt
- Shadow Claw / Night Shade / Foul Play
I know that Banette won't be able to get Pain Split or Foul Play until Pokebank is open, but this set is still useful for once it does. Mega Banette is the only Pokemon to have the ability Prankster along with being able to learn Disable, and therefor should be feared. Do keep in mind that Prankster only takes effect the turn after Mega Evolution.
The point of this moveset is to set up a Substitute as early as possible, then Disable the opposing Pokemon as soon as they use their move of choice to try and break your Substitute. Pain Split can be used to restore HP if multiple Subs are needed, though Taunt can be used to keep entry hazards from being set up. The last move can be one of the offensive moves of your choice, with Night Shade and Foul Play seeming more useful; although Shadow Claw is still viable, since even with no investment, Mega Banette's attack is higher than its HP. You can also swap the offensive move for a status-inflicting move, but this is not suggested.