Gym Leader: Brock (Pewter City)

Pokémon:

• Geodude (Level 10): Rock/Ground
• Onix (Level 12): Rock/Ground

Weaknesses:

Both Geodude and Onix are Rock/Ground types, making them 4x weak to Grass and Water-type moves, and 2x weak to Fighting and Ground-type moves. Electric, Fire, Flying, and Bug moves are not very effective.

Strategy:

Your starter Pikachu will struggle here as Electric moves are not effective. Your best bet is to catch a Mankey or Nidoran♀/Nidoran♂ on Route 22 (west of Viridian City). Train them until they learn a Fighting-type move:

• Mankey: Learns 'Low Kick' at Level 9.
• Nidoran♀/Nidoran♂: Learn 'Double Kick' at Level 12.

These Fighting-type moves will be super effective. Alternatively, if you chose Bulbasaur (via trade or glitch), its Grass-type moves would also be super effective. Use Potions to keep your Pokémon healthy. Focus on hitting their weaknesses and you'll win the Boulder Badge.

Reward:

Boulder Badge, TM34 (Bide)