Playing Fire Red!

What are their levels? Charmander learns Metal Claw at 14 if memory serves, which was something they did to give Charmander users a bone against Brock. However, his Rock types know Rock type moves in FRLG so he can KO Char if you’re unlucky along with Butterfree(another common alternative to the Water/Grass starter via its Confusion attack but it takes x4 from Rock moves). You can also utilize Ember but remember it’s resisted but can also burn Brock’s Rock types so if you manage that, then the power of his moves drops by 50%. Fortunately Onix is one of the worst Rock types from an offensive standpoint. Great Defense though, hence why Bulbasaur and Squirtle have such an easy time against him since Special Defense is very poor. You can also utilize Mankey against him because Fighting > Rock. Mankey learns Low Kick at 6 and Karate Chop at 11, the latter of which has higher Crit ratio. You can also utilize the Nidoran lines(male or female) as they both learn Double Kick at 12 like they do in Yellow.

Another bold and risky tactic is to utilize Pidgey and its Sand Attack to lower their accuracy. Then you can utilize Pokémon that are a risk to use(like Char) and use that to take him down safely with either Metal Claw or Ember.