Ok, Charmander is a fire-type. And Brock`s pokemon are rock-types. Try using growl first, then keep going with metal claw. If you need anymore help, just ask. I`m very good at pokemon. Player since `99.
ok you should catch like 5 caterpies or pidgeys and swithch out your charmander with them wen he's HP is close to red and restor he's health then hope for the best. ps. i hate battling brock to he's inposable to bet on yellow.
Yeah, use Metal Claw. You can also use the Pidgey tactic if you have patience (catch the strongest Pidgey you can find, level him up if you need to, and then spam Brock's Pokemon with Sand-attack until they miss constantly, then send your Charmander out to chip away at his health. This is how I beat him on Yellow, with just Pidgey and Pikachu).
a strategy i use to solve this, always catch and train at least three varied pokomon at the start of the game and keep them at about the same level, such as your starter, pidgy, and ralts, this usually lets you have at least several moves that are effective against your opponent