Team Star's Fire Crew has its base in East Province (Area One).
How to Find the Fire Crew Base
From the east side of Artazon, you can go northesat and follow the path to the Pokémon Center of East Province (Area One). Go west from there, and you will find the Fire Crew Base to the south.
Defeat the Guard at Schedar Squad's Base
At the front gate of the Fire Crew's Base, you are stopped by the guard. If you are ready to battle, tell the guard you're not turning back. The Team Star Grunt has a level 25 Houndour. You get 1300 Pokédollars for winning.
After the battle, there will be a cutscene.
Enter Schedar Squad's Base
If you have defeated the other four bosses of Team Star already, be aware that after you defeat the fifth boss of Team Star, the Academy entrance will be blocked by a Pokémon trainer with high level pokémon.
When you're ready, go to the door of the base and ring the bell. However, you can only bring the first three Pokémon in your party.
About the Star Barrage
Team Star's fighting style is called the Star Barrage. You need to press R to send out multiple Pokémon at a time. You can go back to Clive to heal up your Pokémon. You can also use vending machines to help recover your Pokémons' HP, if you find one. You need to defeat 30 of Team Star's Pokémon within 10 minutes to make Team Star's boss come out to battle you. If you need to exit the challenge early, press Y.
During the countdown, move through the base and approach Team Star Grunts to make them send out their Pokémon. Face the enemy Pokémon and press R to send out your Pokémon against them. Your Pokémon will automatically battle them. They will do better if their type is supereffective against the enemy Pokémon's type. You can press ZR to recall your Pokémon back to you if needed.
Defeat Mela, Boss of Team Star's Fire Crew
After your Pokémon defeat 30 of the enemy's Pokémon, Mela will challenge you to a regular battle, where you can use the six Pokémon in your party.
Mela has a level 27 Torkoal, followed by a level 26 Fire-type Revavroom (the Schedar Starmobile).
Afterward, there will be a cutscene where you learn more about Team Star.
You get Team Star's Fire Crew Badge, and TM038 Flame Charge. (Press A if nothing is happening.) You'll also get some LP, and more TMs will be unlocked in the TM Machine. You will also receive Pokémon materials that you can use to make TMs.
Explore the Base
After this, you can freely explore the Fire Crew Base, where you will find various items, including a Fire Tera Shard in the west side of the base, and a TM008 Fire Fang behind the big tent in the southwest part of the base.
If you want to play the Star Barrage again, you can ring the bell at the front gate of the base.