The gentle Erika has an all-Grass team, so it’s a good idea to face her with Fire or Flying-type monsters.
The battle begins with Tangela, the only pure Grass-type here. It can use Power Whip and Mega Drain for STAB–the latter also stealing HP. Plus Sludge Bomb, and Reflect to weaken physical attacks. In addition to Fire and Flying, Poison and Bug are fine choices too.
Afterwards, Erika can send out her Exeggutor. This one carries Solar Beam and Psychic for STAB–the former requiring a turn to charge up. It also knows Sludge Bomb and Hyper Beam. If you’re not afraid of Psychic, Bug-types can be very useful here because of their quadruple damage. Otherwise, Dark is a great choice.
Parasect can either be a massive nuisance or gone in the blink of an eye. It can use Leech Life and Solar Beam for STAB–the former steals HP, while the latter requires charging. Plus Brick Break to remove barries, and Spore to guarantee sleep. Fire or Flying-types are highly recommended to one-shot it before it uses Spore.
Vileplume is Erika’s choice Pokémon. It can now use Petal Dance and Sludge Bomb for STAB–the former causes confusion to itself after a few turns. Plus Moonblast and Stun Spore. Besides Fire and Flying, Psychic is also a stellar choice.
Finally, Erika has evolved her Weepinbell into Victreebel. Now it can use Power Whip and Poison Jab for physical STAB, Swords Dance to greatly boost its Attack, and Reflect to protect its team from physical attacks. Similar to Vileplume, Fire, Flying and Psychic are the best choices offensively and defensively.