Since Piers is the seventh Pokemon Gym Leader found in both Pokemon Sword & Shield, defeating him will be no small task. Even if Piers can't Dynamax or Gigantamax any of his Pokemon, he will still be very difficult to overcome in battle. Like a number of the Gym Leaders before him, all of Piers' Pokemon will be fully evolved. If that's not bad enough, he will also bring four Pokemon into battle, and all of them will be bulky Dark Type Pokemon.
Thankfully, like all Gym Leaders, Piers can be much more manageable to defeat should you make smart decisions before leading up to battling him. One thing you can do is choose Scorbunny as your Starter Pokemon way back when your journey first begins and fully evolve it by the time you reach Piers. While leveling up and evolving, Scorbunny's Evolution Chain actually can learn a few Fighting Type Moves. For the most part, Piers team is weak against these very types of attacks.
Even if you don't select Scorbunny to journey through The Galar Region with, there are still a few things one can do leading up to the Spikemuth Pokemon Gym to better deal with Piers' Dark Types. One of these things is to capture Impidimp when you go through Glimwood Tangle and quickly evolve it twice into Morgrem and then Grimmsnarl, respectively. Grimmsnarl is a dual Dark/Fairy Type Pokemon, meaning it won't take much damage from Piers' Dark Type Attacks while still dealing massive damage with its Fairy Type Attacks (since Dark Types are also weak to Fairy Type Moves).
But should you not choose Scorbunny as your Starter Pokemon or have the desire to catch and evolve Impidimp into Grimmsnarl, like all Pokemon Gyms you don't actually have to in order to defeat him. Piers' four Pokemon will all be below Level 50, so one could, honestly, train all of their Pokemon to Level 50 or Level 55 and still defeat him. This is incredibly simple to do if you battle all the trainers throughout your journey and participate in plenty of Max Raid Battles back in the Wild Area (and stock up on EXP Candy when you do).
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