Walkthrough:
By heading to the Hulbury Gym next with Piers and Hop, you will learn from Nessa that a Pokemon is currently Dynamaxed within the Stadium of her Pokemon Gym. The three of you will then team with Nessa to bring down the Dynamaxed Pokemon in a Max Raid Battle. This time, the Dynamaxed Pokemon will be a Level 62 Gyarados, one that will know the moves Waterfall, Stone Edge, Crunch, and Avalanche (also being able to use the Max Moves associated with each of those attacks).
After helping her calm down the Dynamaxed Pokemon, Nessa will gift you with her Rare League Card.