The fourth Gym Leader you will face on the road to collecting all eight Kanto Gym Badges is Erika. She is the Celadon City Gym Leader, specializing in Grass Type Pokemon. Much like Lt. Surge, Erika's Pokemon Team will be a little more difficult to defeat since you are practically halfway through with the Kanto Pokemon Gyms. If you did pick Charmander as your Starter Pokemon, you will have a clear advantage over Erika since she is especially weak against Fire Types.
If you happened to choose Bulbasaur or Squirtle as your Starter Pokemon, you might need to add an additional Pokemon or two on your way to Celadon City to better help you with Erika's Victreebel, Tangela and Vileplume. The best Pokemon for this job is either Pidgey (found on almost every route leading up to Celadon City) or Flareon (obtained by evolving the Gift Pokemon Eevee you can obtain in Celadon City with a Fire Stone), since Erika's Grass Type Pokemon are weak against both Flying Types like Pidgey and Fire Types like Flareon.
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