I'm playing a hacked version of FireRed that makes it like Yellow and I only have Pikachu level 10 and a Level 3 Caterpie. Help please.
P.S. All moves up to Gen 6 are available.
There are a few ways:
Get your Pokemon to 10+ levels: This makes last ditch attempts of winning more likely to succeed.
Keep Caterpie as far away as possible: Caterpie can't even touch Geodude or Onix, while they can seriously dent it with their own Rock-type moves. Otherwise fully evolve it into Butterfree and teach it Giga Drain if possible. That should seal the game.
Catch a Nidoran (Male) and make it learn Double Kick: you get some in the locality. It gets Double Kick at level 12, and can be very handy in taking down Brock's Rock types.
Pikachu should learn Iron Tail: This option is not possible but you said you are playing a hack, so I don't really know. Hence I suggested this, as Rock Types will hate this move.
Hope I helped!
Evolve caterpie to metapod then butterfree and lead with it. Use confusion until geodude faints, then send out pikachu on onix and use growl 6 times to lower its attack to the minimum, then switch to butterfree and defeat onix with confusion
That should work
Don't forget to buy potions
Hope I helped