I'm not past Brock yet. I did a Red play through a few years back using Charmander and it was very easy after you get out of Mt. Moon.
Brock is hard for a few reasons. Rock and Ground types take 50% damage form normal and fire type attacks so Charmander will have trouble taking them down. Also fire types take 50% more damage from ground type attacks. So you have to put together a team to beat Brock. The only pokemon available from the area are Pidgey, Weedle, Catapee, and Ratta. Their attacks are all normal types so they do 50% damage against Brock's pokemon. So it is hard.
Misty is easy since her water types take normal damage from normal type attacks. You are also given a wide variety of pokemon to capture after Mt. moon
Brock is hard for a few reasons. Rock and Ground types take 50% damage form normal and fire type attacks so Charmander will have trouble taking them down. Also fire types take 50% more damage from ground type attacks. So you have to put together a team to beat Brock. The only pokemon available from the area are Pidgey, Weedle, Catapee, and Ratta. Their attacks are all normal types so they do 50% damage against Brock's pokemon. So it is hard.
Misty is easy since her water types take normal damage from normal type attacks. You are also given a wide variety of pokemon to capture after Mt. moon