Now for the main attraction! Brock has a pair of high Level and very sturdy Pokémon. His first Pokémon, Geodude, you’ll have fought previously. This one is similar, but higher Level. We recommend leading with your Grass-type so they can quickly take it out with their super-effective moves.
Definitely don’t use Pikachu’s Thunder Shock since it’s a Ground-type. Your partner’s Double Kick is worth a shot, but Geodude has pretty bulky Defense, and your partner won’t get STAB (same Type attack bonus) for using it, so it likely won’t do as much damage as a Grass-type would.
Next up is Onix, Brock’s signature Pokémon of sorts. This one can use Bind to inflict residual damage and Headbutt that can make your Pokémon flinch if they’re slow. Hopefully your Grass-type survived Geodude so they can continue to unleash their super-effective hits.
Succeed in smashing Brock’s Rock-type Pokémon and he’ll grant you the Boulder Badge as well as his new favourite TM01 Headbutt. Originally, he gave you the TM for Bide, but that move wasn’t very useful in the hands of the player. If you have Eevee, teach it Headbutt ASAP as it can get a lot of mileage out of it.
Outside the Gym, somebody will stop you to say “hi”. Why, it’s Blue, who you may recognize as Gary from the Pokémon cartoon. For those unacquainted, he’s the confident grandson of Professor Oak and was the original rival in Red, Blue and Yellow. In this universe however, he seems to be just another Pokémon trainer. He’ll give you 5 Great Balls. How oddly kind of him.
When the coast is clear, return to the Pokémon Center if necessary, then make your way east towards Route 3.