After you go down Jagged Pass, you arrive in Lavaridge Town, where you can challenge the Fire-type Gym and its leader Flannery.
Fire-type pokémon are weak to Water, Ground, and Rock moves. Bug, Grass, Ice, and Steel pokémon are weak to Fire attacks.
This gym has wooden floor panels on the upper level that you can step on to fall to the lower level. On the lower level, you can stand on geysers to go back up to the upper level.
First, go forward and step on the wooden panel north of the entrance. Then go to the right and step on the wooden panel there.
To the north, you can battle Kindler Cole, who has Numel and Slugma.
Go north and step on the wooden panel there. On the lower floor, a trainer is hiding between the two leftmost wooden panels. He is Ninja Boy Hiromichi, who has Koffing.
Step on the leftmost wooden panel.
Kindler Axle is running around nearby. He has a Slugma.
Go all the way north, where you can fight Battle Girl Sadie. She has a Meditite.
Step on the wooden panel near the Battle Girl. Go to the right, where Ninja Boy Shoji is hiding between the two rightmost panels. He has a Koffing.
Then step on the rightmost panel. To the south, you can battle Ace Trainer Zane, who has Slugma and Kecleon.
After battling Zane, step on the panel south of him.
In the room where you end up, you must battle Kindler Andy, who has a Numel.
Then step on the panel on the left to reach the gym leader.
Flannery has Slugma, Numel, and Torkoal. All are weak to Water and Ground moves. Slugma and Torkoal are weak to Rock moves as well. Numel is especially weak to Water. All three know the move Overheat, which is a very powerful Fire attack.
After you Win
After you get the Heat Badge, traded pokémon up to level 50 will obey you. You will also be able to use Strength outside of battle, which allows you to explore the rest of Fiery Path.
Flannery also gives you TM50 Overheat.
To leave the gym, just go south and jump down the ledges.
Outside, your neighbor shows up and gives you the Go-Goggles. With these, you will be able to enter the desert in Route 111, where you can get a fossil that you can take to Rustboro City to turn into a pokémon. You can also battle trainers and find Secret Spots in the desert.
Your neighbor asks if you want to go to Petalburg City right away. You can say no if you want to explore the new areas that have opened up. Petalburg City can be reached by going to Rustboro City, then going south and east from there to Petalburg City, where you can challenge the Petalburg Gym.