I've just tested T2 Banette vs. Mario. Mario has no chance at all. Banette is too fast, and Mario is weak. Well, that was obvious, wasn't it? But I thought I'd report anyway. BTW, the record was 3-0 straight Banette, and 1-0 Banette/Sableye.
My list is very fast, and I don't use DRE at all. Buffer Piece is terrific.. not a "must" but darn near. I also run 2 Cess which kept the Lucario Lv.X from ever having even a chance. I don't run Mentor, but Roseanne's so I am assured of good Energy draw, which the DRE loss could affect, but doesn't for me. I run 4 TV Reporter, but no Adventurer. It's overkill, and Banette is so fast it's ridiculous.
You need 3 Night Maintenance and *no* TSD since you must get back Energy, usually gone 2 at a time.
I thought about Banette/Blissey today also. Banette gets back Banette, and Blissey gets back Energy for use. I'd probably use 2-2 Blissey just for cleanup and so it wouldn't get in the way of the Banette.
Gengar isn't really a good choice for Banette (too slow, and the current Gastly is terrible). If anything, Gengar/Crobat, or Gengar/Muk. Just IMO>