Thanks for the help, but I actually ended up defeating him just a bit ago!
I ended up swapping out Meowstic for a Sandslash and used it to take down Gengar. With a Synthetic Seed, Sandslash would always survive the first Shadow Ball, and OHKOs with Dig. Toxicroak 2HKO'd Rotom *just* barely via Sucker Punch, and Delphox could sweep through Doublade, Dhelmise and Banette with no effort.