I personally think Banette can be used very well, though mega evolution is kinda worth the stat increase, the ability it brings is nothing more than precautionary, I recommend using Him with toxic spikes set out and training the Sp.atk, Sp. Def, and HP EV's. moves that work well with this build could definitely be
Hex (does more damage if the pokemon has a status condition doing 130 dmg when the status condition is on the rival pokemon, it's almost made for banette, this pokemon has a lot of moves that can be really annoying)
Trick Room (This move works really well for banette given his insanely terrible base speed stat, use a brave natured banette to up the physical attack and down the speed even further)
Will-o-wisp/ Toxic/ really any status effect move here that banette can use (though I recommend will-o-wisp, I use this slot to make sure a status effect always stays a possibility, if the pokemon has levitate or something like that the mega evolution ability that banette gets (prankster) gives banette priority with status effect moves, will-o-wisp works well because it can cripple a physical attacker pokemon a tad bit)
Shadow Claw ( or any stab move) (Banette has an insanely good base attack stat using a stab move with banette has no question about it this can be used as a last resort or if you wanna change it up and not spam hex the entire time this is most certainly a choice)
(that's my personal build for banette I apologize if It's a bad moveset, I personally think it could work and friends of mine who have been into competitive battling seem to think it's a decent idea so I guess that about it)