Walkthrough
By heading to the Circhester Gym next with Piers and Hop, you will learn from either Gordie or Melony (depending on the version you're playing) that multiple Pokemon are also Dynamaxing at their Gym because of Sordward and Shielbert. Because of this, each of you will have to defeat one of the Dynamaxed Pokemon in a Max Raid Battle all by yourself. In Pokemon Sword, the Dynamaxed Pokemon will be a Level 62 Gigalith, one that will know the moves Rock Slide, Earthquake, Throat Chop, and Stone Edge (also being able to use the Max Moves associated with each of those attacks). In Pokemon Shield, the Dynamaxed Pokemon will be a Level 62 Froslass, one that will know the moves Thunder, Psychic, Shadow Ball, and Icy Wind (also being able to use the Max Moves associated with each of those attacks).
After helping clear the Circhester Gym of all the Dynamaxed Pokemon, you will receive either Gordie's/Melony's Rare League Card.