do you have a meowstic? A male meowstic with Prankster can set up Reflect or light screen on the first turn so your pokemon might have a higher chance to survive, they can be found at the aqua gang hideout. Also what abilities does your Nidoking has? if it doesn't have sheer force use and ability capsule on it, and I know that Serperior is your starter pokemon but right now it's a liability in this gym so I suggest you should change it to another pokemon that has a type advantage. You can get an Alolan Raichu if you want, not only it is a Psychic type it is also fast so it can outspeed most of her pokemon. If you don't know where to get it, you can find a Pichu on top of a building at Onyx Ward. Just talk to the ace trainer and he will let you in, the Pichu is hiding behind a tree so look carefully. Then once it evolve into Pikachu take it to Apohyll to evolve it, it will automatically learn psychic. Hope that helps