After a long journey south through Route 9 and a battle with Marnie, you reach Spikemuth.
You can visit the Spikemuth Pokémon Center if you wish. Then go east, and there will be a cutscene. You will get Piers's League Card.
Spikemuth Gym Mission
Afterward, you will have to change into your uniform, signalling the start of the Gym Challenge.
Go east and you will battle a Team Yell Grunt who has a level 42 Galarian Linoone. You get 3360 Pokédollars for winning.
Go east and you will find a Mr. Mime blocking the way. Check on the invisible barrier, then try to go west and you will battle a Team Yell Grunt who has a level 42 Thievul. You get 3360 Pokédollars for winning.
Keep going east and you will battle a Team Yell Grunt who has a level 42 Scrafty. You get 3360 Pokédollars for winning.
Nearby, you find a Max Revive.
Go east and you find another Mr. Mime. Check on the invisibile barrier, then go west and you will battle a Team Yell Grunt who has a level 43 Weavile. You get 3440 Pokédollars for winning.
Go east, and when the path is blocked, go north into the building and go east. You will battle a Team Yell Grunt and Gym Trainer Joshua. They have a level 43 Drapion and a level 43 Liepard. You get 7568 Pokédollars for winning.
Go east and approach Marnie and there will be a cutscene.
Keep going east, and if you go to the northwest corner of the arena area, you can talk to the grunt at the stall to trade an Obstagoon to receive Edmund's Kantonian Mr. Mime, nicknamed Mymo.
Go east and there will be a cutscene. Then you will battle the Gym Leader.
Gym Leader Piers
In this battle, you and Piers can't Dynamax your pokémon. Gym Leader Piers has a level 44 Scrafty, a level 45 Malamar, a level 46 Obstagoon, and a level 45 Skuntank. You get 7360 Pokédollars for winning.
Afterward, you get the Dark Badge. You can now catch Pokémon of up to level 55. You also get a Dark Uniform, and TM85 Snarl.
At Poké Marts, you can now purchase Max Potions and Max Repels.
You will automatically go to Route 9, where there will be a cutscene.
You can go east to get back into Spikemuth, and if you go all the way east to the area with the stage, you can go south from the stage to find a pair of Choice Specs.
When you are ready, go west from Spikemuth into the Route 9 Tunnel. Go west through the tunnel, past the crowd of people, and you will reach Route 7.
Go west and there will be a cutscene.
Go west again, and if you haven't already, you will battle Gentleman Caden, who has a level 44 Doublade, a level 44 Passimian, and a level 44 Polteageist. You get 8800 Pokédollars for winning.
Northeast of the gentleman, if you didn't get it already, you find TM58 Assurance up against the railing.
Nearby, you will find Renee's Pokémon Camp. If you didn't go here earlier, this adds the camp as a Flying Taxi destination.
If you go south from there, then east, you find a ramp just south of the bridge that leads to an area with a pair of Safety Goggles on the east side and a berry tree on the west side. If you keep shaking the berry tree, you can have a wild pokémon encounter with Greedent.
From there, go west to Hammerlocke, and approach the stadium. There will be a cutscene.
When you're ready, go north into Hammerlocke Stadium.