You probably noticed it before entering, but the Pokémon Centre is directly in front when you arrive from the side entrance. You might as well let yourself in. After making sure your Pokémon are healthy and you have all the potions you need, head east from the Pokémon Centre. If you go west, you'll reach the shutters. Approach Marnie and she'll give you Piers's League Card.
Know Your Match-Ups: Dark
Dark-types are weak to Bug, Fighting and Fairy. On the other hand, they're strong against Psychic and Ghost-types. They also resist Ghost and Dark moves and, crucially, they take zero damage from Psychic moves.
For this "gym", we recommend Fighting and Fairy-types. If you're lacking strong Pokémon of these types, you can catch Level 50 Grapploct from Route 9. A Ground or Steel-type would be useful for backup as well. There's a Steelix you catch in the Giant's Mirror, near the border to the Stony Wilderness.
As it happens, this is also the beginning of the Gym mission. All you have to do is head all the way east, while dealing with the local troublemakers. Nothing complicated at all. If you're not ready yet, you can retire and come back when you're prepared. Anyway, you'll meet your first roadblock within a couple of house blocks.
Team Yell Grunt
One of Team Yell's favourite Pokémon. If you don't remember its weaknesses by now, they're Fighting, Bug and Fairy. In particular, it has a double weakness to Fighting. If you can't score a one-hit KO, avoid physical attacks as it could use Counter.
Next, continue east until you run past two Team Yell Grunts who seem pre-occupied and a Mr. Mime blocking the way. Try to go past the Mr. Mime, then turn around. The Grunts will finally notice you.
Team Yell Grunt
Another Team Yell fave. This evolved Nickit is a pure Dark-type, so you ought to know what to use. Be wary of Sucker Punch, which can go first if you choose an attacking move.
A few more house blocks later and two Grunts will leap at you from the windows above. Do not attempt this home! Ouch, their poor legs...
Team Yell Grunt
This Sword exclusive Pokémon takes 4x damage from Fairy and 2x damage from Flying and Fighting. Its most dangerous move is Swagger, which can increase your Pokémon's Attack and cause confusion (and damage dealt via confusion is based off your Attack, so...).
Once dumb and dumber are gone, pick up the nearby Max Revive on the right. Then continue on your way. After several house blocks, you'll come across another Mr. Mime. Again, turn around to leave and some Grunts will try to jump you.
Team Yell Grunt
Weavile is an evolved Sneasel. It retains the same typing, so it takes 4x damage from Fighting and 2x damage from Fire, Rock, Bug, Steel and Fairy. This one can use Ice Shard to sneak in an attack before you and Metal Claw to damage Rock and Fairy-types.
A few more house blocks later, there will be some rubble blocking the entire street. Go inside the building above, then pass through it towards the east. When you're about halfway through, the final pair of Grunts will get in your way.
Team Yell Grunt and Gym Trainer Joshua
For some reason, this is the only Double Battle, despite every Grunt being in a pair. Anyway, Liepard is pretty easy to suss out. Of note, it can use Taunt to stop your Pokémon from using status moves
Meanwhile, Drapion is weak to Ground only. If you have Steel-types though, you can easily tank it, because it mainly focuses on Poison-types moves.
Exit via the doorway on the right. You'll find Marnie up ahead. After she gives Team Yell an earful, head past them towards the courtyard. Before you go elsewhere, head for the top-left corner. Over here, there's a Grunt who wants to trade his Kantonian Mr. Mime for your Obstagoon. This is the only way to get an original Mr. Mime until Pokémon Home arrives.
To get an Obstagoon, you can catch a Galarian Linoone in Bridge Field, in the Wild Area. Then level it up during night time to evolve it. In any case, approach the crowd towards the right to trigger the Gym Leader battle.
Gym Leader Piers
Piers's first Pokémon is Scrafty. You already faced one moments earlier, but if you forgot, it takes 4x damage from Fairy and 2x damage from Fighting and Flying. His one can use Fake Out to cause a flinch on the first turn. Physical attackers should be wary of its Intimidate ability as well.
Next, he may send out Malamar, which is an evolved Inkay. It takes 4x damage from Bug and 2x damage from Fairy. Do not try to lower its stats, because its Contrary ability will boost its stats instead. For attacks, it can use Foul Play to deal damage based on your Pokémon's Attack stat.
His third mon is Skuntank, an evolved Stunky. Like Drapion, it's weak to Ground only, but Steel-types are a good defensive option. Watch out for Toxic, which causes poison that gets worse each turn, and Sucker Punch. Also, if it gets KO'd by a contact move, your Pokémon will be damaged by its Aftermath ability.
Finally, he has an Obstagoon, which is an evolved Galarian Linoone. It has the same weaknesses as its previous forms, so it takes 4x damage from Fighting and 2x damage from Bug and Fairy. It loves to use Obstruct, which protects it from damage and greatly lowers Defence if hit by a contact move.
Just in case you missed it, neither you nor your opponent can Dynamax or Gigantamax during this Gym Leader battle, as Spikemuth lacks a power spot necessary to do so.
After silencing Piers, you'll receive his Dark Badge as well as a Dark Uniform. Back at the entrance, you'll also be given TM85 Snarl. Next, you'll suddenly be dragged to Route 8, just outside the (now open) main entrance to Spikemuth. Leon will show up and run off ahead. As soon as you're prepared, head all the way west to the Route 9 Tunnel.