I'd recommend doing the Viridian Forest clearing mini-quest, to gain access to a secret area where grass-types can be obtained, which possess a 4x advantage against Brock's Pokemon. The starting point of this quest is in the southwest corner of Viridian City, and the quest entails obtaining the caught data entry of Butterfree into your Pokedex. Then it rewards access to the Viridian Forest Clearing. For an entire list of spawns available there, /list viridian forest clearing. An additional reward of the mini-quest is 3 reputation points.
Obtaining a Butterfree can be a bit time-consuming, so I would also suggest hunting a mankey west of Viridian City, which possesses a 2x advantage against rock-types.