FISHIOUS REND MEGA SHARPEDO
A couple of things. Banette's bulk is pretty bad, in fact it is statistically the frailest of all available Ghost-types, which would by extension make it no different from any other offensive Ghost-type you can find.it's bulk isn't that bad
He is unique in this way. If you want to use a Ghost-Type Pokemon but are tired of the frail but hard hitters Banette is who you go to
Major Battles: You can him late, so he is only around for:
Drayden - Provides some good utility with Trick Room and Will-O-Wisp, it can hit really hard with Shadow Claw against his weaker Dragons.
Team Plasma Shadows - Seriously. No.
Marlon - Can take care of Jellicent with Sucker Punch. Carracosta wrecks him with Crunch, though.
Team Plasma - Fairly normal to be honest. Use him if you want to, he's not particularly good though.
Additional Comments: Is unique so is good if you want a change of pace.
You cannot get Trick Room until after Ghetsis (N gives you HM08 Waterfall to reach the Abundant Shrine), and you didn't even list the E4 matchups (hint: he kinda sucks, even against Caitlin due to bad Speed and bulk). Banette also trades badly with even Team Plasma grunts, not OHKOing anything that cannot outright OHKO Banette back.