The Lavaridge Gym is home to many Fire-type pokémon, which are weak to Water, Ground, and Rock moves. Flannery is the Gym Leader.
This gym is full of holes that you have to either fall into or stand on to be elevated by geysers. In addition, there are some holes that a trainer will pop out of from a geyser.
The trainers here have Numel, Slugma, Meditite, and Kecleon.
Path in Ruby and Sapphire
To go through the gym in Ruby and Sapphire, go into the hole on the left. (If you go to the one on the right, a trainer comes out).
Down below, go into the hole on the left.
Up above, there will be a trainer if you go to the upper-right hole. Go into the upper-left hole to proceed.
Step on the hole. There is a trainer in the southeast hole. Fall into the upper left hole. Go north and step on the northern hole.
In the upper room, the northwest hole has a trainer. Fall down the northeast hole.
Go to the east and walk onto the hole there. In the room up above, the northwest hole has a trainer. Use the southwest hole to proceed.
Go south and enter the hole. You will be in the Gym Leader's room.
Path in Emerald
The hole near the entrance leads to a dead end where you can fight a trainer.
From the Gym entrance, go northwest to find a hole. Fall down it. Go south and follow the path to find two trainers. You have to go past them to proceed. After you pass those trainers, you can go into either of the two holes.
Up above, you can have a double battle with the two trainers there. Then go north and battle the trainer there if you wish. Then jump into the northwest hole.
Go north and battle the trainer if you wish, then step on the northwest hole.
Up above, fall into the northwest hole. Then down below, go east and step on the first hole you find. From there, fall into the southeast hole. Jump down one ledge, and step on the hole there. You will be in the Gym Leader's room.
In Ruby and Sapphire, Flannery has two Slugmas and a Torkoal. In Emerald, she has a Numel, Slugma, Camerupt, and Torkoal. Several of the opponents have Overheat and Sunny Day. Overheat is a very powerful attack, but it sharply decreases the pokémon's Sp. Attack, meaning that the move becomes less and less powerful throughout the battle. Sunny Day increases Fire attack strength and reduces Water attack strength.
After the battle, you get the Heat Badge, which allows you to use Strength outside of battle, and TM50 Overheat.
Visit the Desert
After you leave the gym, your neighbor gives you the Go-Goggles. Now you can go into the desert in Route 111. You can go east from Lavaridge and jump down the ledges. Be sure to get the Nugget on the way down.
If you go back into Fiery Path, you can use Strength to access a new area. You can find a Fire Stone to the south and TM06 Toxic to the north.
In the desert, there are several trainers, as well as a hidden Protein, two hidden Stardusts, and a Rare Candy. You can also find TM37 Sandstorm to the south.
If you have the Mach Bike, you can ride up a slope in the northern cliff and walk around the upper area, where there are some Secret Base spots and a trainer.
In the northeast corner of the desert there are two fossils in the sand. You can only take one of the fossils: the other one will sink into the sand. After you get a fossil, take it to the Devon Researcher in Rustboro City and he will turn it into a pokémon. The fossil on the left is the Root Fossil, which will turn into Lileep. The fossil on the right is the Claw Fossil, which will turn into Anorith. Lileep is a Rock/Grass pokémon. Anorith is Rock/Bug.
Now that you have four badges, you can battle your father in Petalburg Gym.