If this is Red and Blue, then both Geodude and Onix do not have a Rock move, so you are very safe to use any one of the Pokemon you have on your given team.
I would recommend using butterfree.
Brock's ace Pokemon Onix is at level 14. The moveset with moves Butterfree learns at level 14 or below is given below:
-Harden(Learnt when Butterfree is a Metapod)
-Poison Powder(At level 13)
-Stun Spore(At level 14)
-Confusion(At level 10 or when it evolves)
Harden is for tanking the hits from Geodude and Onix as the only attacking moves they have are Tackle and Bide. Poison Powder is for Poison. Stun Spore isn't really useful against Brock. Confusion is an attacking move.
First use Poison Powder on the Geodude, then keep on using Harden until you have +6 Defense. Then Spam Confusion until the Geodude Faints. Use Confusion until the Onix uses Bide. After Onix uses Bide, then use Poison Powder, and then any non-attacking move. Do this until Onix faints of the poison. In case Butterfree is at low health then switch out to something else.
There is a drawback to this though, this strategy is vulnerable to critical hits(though Geodude and Onix are very slow, and thus have a very low chance to crit because in Gen 1, critical hits were based on the speed of the Pokemon)
I hope this works well for you and good luck in your nuzlocke!
Sources:
https://bulbapedia.bulbagarden.net/wiki/Brock
