If you evolve it at level 50, it will ask if you want to learn Trick during the battle, then afterward, it will evolve. As for a set, I would recommend one of these moves:
Shadow Claw: Better because of higher attack.
Will-o-wisp: Gives the foe a burn, which also lowers attack.
Payback: Banette doesn't have the best Speed, so this will work well.
Thunderbolt: Good move to have. Surprise move.