>>27193824That Gengar set pairs well with a 'mon that knows Encore, as you can force them to use one move and then Disable it, forcing the 'mon to struggle. Liepard Whimsicott and Infernape are good partners for this. The Whimsicott and Liepard can learn Fake Tears as well, which sharply lowers Sp. Def and then Gengar can follow that up with Sludge Bomb.
For a sixth 'mon for your team? I think one of the three I just mentioned would go great, maybe use Infernape since it is more straight forward and use Fake-Out Overheat Close Combat and Encore with it. Have fun.