So, because I am trying to knock out another achievement, collecting all the TMs, I wanted to tackle Erika first, as she is one of the only Kanto leaders to give you a TM. I forgot Janine does as well, but oh well.
My original thought was Jumpluff, as it is Grass/Flying. But another Pokemon came to mind as I was looking through my boxes.
Paras/Parasect. A bug/grass type, which would provide a 4x resistance to Erika's grass attacks. Jumpluff would have the same as a Grass/Flying, but Parasect has Slash, a really good normal type move with a high crit rate.
Combined with having Parasect hold Leftovers and being quite tanky, this meant that most of Erika's attacks didn't do enough damage to bypass my healing.
Victreebell was an exception, with Acid, but... This is what the move Spore is for. Victreebell might have been faster, but it was also a deep sleeper.
So with that, Parasect swept the Grass gym.
Thanks buddy!