Enter the Pewter City Gym. There's one junior trainer you can battle before Brock to gain some extra experience. Once ready, challenge Brock. He will use:

- Geodude (Level 10): Rock/Ground type.
- Onix (Level 12): Rock/Ground type.
Focus your attacks with your Fighting-type Pokémon (Nidoran with Double Kick or Mankey with Low Kick). If you don't have a Fighting-type, you can try to chip away with Normal-type moves from Pikachu or other Pokémon, but it will be a tough fight. Potions are your friend here. After defeating Brock, he will award you the Boulder Badge and TM34 (Bide).