Pretty sure you won't be able to brute force your way through it. I tried using Gigalith too thinking I'll just spam Rock Slide and resist all the poison lololo except that didn't work at all because she has a lot of super effective moves on her team and I never really got a chance to use it.
Looking at what you have though, can you get Sucker Punch back on Toxicroak somehow? You'd probably be able to deal with Gengar easily with it. If not, Drapion would be your best bet in that area. Try to use Gardevoir in the beginning because it'll just get one-shotted by Drapion or Gengar later on anyway. If you haven't seen her full team yet, I believe it's Muk, Tentacruel, Nidoqueen, Gengar, Drapion, and Seviper- maybe that'll help you plan ahead or something. I won 2-0 so it's probably going to be close no matter what you do, unless you're Inuki. Trying to think of things that aren't really far out of the way and don't need tons of training...
As far as I remember, Magneton is really good here, as a way to deal with Tentacruel plus the poison immunity- all the better if you manage to get off Thunder Wave on Gengar, because that should allow Gardevoir to come in and snipe it. That's basically exactly what I did, except I had Lanturn instead and Mawile as a Steel-type. You miiiiiight have to get lucky to dodge a Focus Blast, but it's worth a shot, I think. Drapion is probably going to be the biggest issue because of the Dark-type and its sole weakness is Ground, something I don't think anyone reliably has covered at this point. You should be able to handle Nidoqueen with Samurott, but idk for sure.
Might edit with more but for now there's that