If Banette has Cotton Guard and this is in-game, don't take it away. Defensive Mega Banette works too, with the following set:
Banette @ Banettite
Ability: Cursed Body -> Prankster
EVs: 252 HP / 4 Atk / 252 SpD
Cotton Guard boosts your Defense to insane levels, and with SpD investment, you're nearly untouchable. Taunt breaks Toxic stalkers, Toxic allows you to put residual damage on the foe, and Shadow Claw is your one attack.