Pokémon:
- Koffing (Level 37): Poison
- Koffing (Level 37): Poison
- Weezing (Level 43): Poison
- Grimer (Level 39): Poison
Weaknesses:
All of Koga's Pokémon are pure Poison-types. They are weak to Ground and Psychic-type moves.
Strategy:
Psychic-type Pokémon are your best bet. If you have an Alakazam (via trade) or a Kadabra, it will be incredibly effective. Other strong Psychic-types include Exeggutor (Grass/Psychic) or even a well-trained Jynx (Ice/Psychic, if traded from Red/Blue). Teach them 'Psychic' (TM29).
Alternatively, a powerful Ground-type Pokémon like Dugtrio, Sandslash, or Rhydon with 'Earthquake' (TM26) will also deal super effective damage. Be cautious of Koga's Pokémon using 'Smog' or 'Sludge' which can poison your Pokémon. Have Antidotes or Full Heals ready. Koga's Gym also has invisible walls, so navigate carefully.
Reward:
Soul Badge, TM06 (Toxic)