Walkthrough:
By heading next to the Motostoke Gym next with Piers and Hop, you will learn from Kabu that a Pokemon is currently Dynamaxed within the Stadium of his Pokemon Gym as well. The three of you will then team up with Kabu to bring down the Dynamaxed Pokemon in a Max Raid Battle. This time, the Dynamaxed Pokemon will be a Level 62 Torkoal, one that will know the moves Lava Plume, Earthquake, Body Press, and Iron Tail (also being able to use the Max Moves associated with each of those attacks).
After helping him calm down the Dynamaxed Pokemon, Kabu will gift you with his Rare League Card.