Use your golem and your mightyena (maybe even swampert)
Golem can hit gengar really hard or at least slow him down with bulldoze so mightyena can kill him. Hes useful agaisnt doublade and rotom as well, so maybe buy some cotton candy to revive him(?)
Same goes for swampert
I usually use sticky web support so mightyena can bite the hell out of him (sewaddle learns it at early lvl 30's, dont evolve it until then)
With moxie your mightyena will probably destroy him until he sents out mimikyu
If he goes for swords dance, you will probably 2 shot him even without sucker punch cause of -1 speed from webs
If he kills mightyena, just go for any mon that can handle him without any buffs, like toxicroak
If you dont want to use web, go for a team that can take some hits, set X-items and try to get his mimikyu out asap