How do you get the sixth badge in Pokemon?

To get the sixth badge in Pokemon FireRed and LeafGreen, you will need a Charmeleon that knows Flamethrower, or a Wartortle that knows Surf. Next, you should have a Graveler that knows Magnitude, for the Ground-Type is stronger than the Poison. Also, bring a lot of Antidotes or Full Heals, just in case your Pokemon gets poisoned. It's also beneficial if you have a Psychic or Steel-Type Pokemon, since they're both good against Poison too. Poison-Type attacks won't affect Steel-Type Pokemon at all!