Korrina specializes in Fighting-type Pokémon. Her team includes a Level 30 Mienfoo, a Level 30 Machoke, and a Level 32 Hawlucha. Fighting types are weak to Flying, Psychic, and Fairy. Your Fletchinder or any Psychic-type Pokémon (like Espurr's evolution, Meowstic) will be highly effective. If you received a Kanto starter, a Charmeleon or Wartortle could also work with Flying/Psychic moves. Hawlucha is Fighting/Flying, making it weak to Electric, Ice, Flying, and Psychic. Use your Mega Lucario if you've obtained it!
