Beating Brock

Brock is the wall Yellow builds on purpose. Both his Pokemon are Rock/Ground, and Ground is immune to Electric, so ThunderShock does literally nothing.

Go west before you go north

Route 22, just west of Viridian City, is open before Viridian Forest. This is the whole solution.

Catch a Mankey, a Nidoran (M), or both

Mankey learns Low Kick at L9 and Nidoran (M) learns Double Kick at L12. Both are Yellow-only learnset changes put there for this exact fight.

Or raise a Butterfree

Caterpie from Viridian Forest evolves at L7 and again at L10, and Butterfree learns Confusion at L12, which handles both of Brock's Pokemon.

Grind to roughly level 12

Viridian Forest trainers plus Route 22 wild Pokemon are enough. Onix has 160 base Defense but only 35 base HP - it folds to a super-effective hit.

Fight

Lead with the Fighting or Bug attacker, keep Pikachu benched, and take Geodude first. Reward: Boulder Badge (+12.5% Attack, Flash usable) and TM34 Bide.