Cotton Guard Banette eh? I like the idea. Could make for a decent physical tank, especially with WoW and Cursed Body.
Banette @ Leftovers
Adamant
Cursed Body
252 HP / 212 Atk / 44 Spe
Cotton Guard
Taunt
WoW / Pain Split
Shadow Sneak / Shadow Claw
Outspeeds minimum speed Skarmory and Taunts it. After one Cotton Guard, Banette's Defense reaches 415, which is just shy of Skarmory's defenses. Might be good in UU?
This might work (or at least make Banette somewhat useful with its new move). However, if you're attempting to outspeed Skarmory, you might want to take into account that the fifth gen analysis recommends running 24 spd ev's on it.