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.

Step 1: Go west before you go north

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

Step 2: 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.

Step 3: 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.

Step 4: 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.

Step 5: 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.