For an opportunity to encounter these Pokémon in a Max Raid Battle, you’ll need to connect your Nintendo Switch system to the internet through Y-Comm or by selecting Mystery Gift in the X menu, then selecting Get the Wild Area News. After that, whenever you see a pillar of light shining from a Pokémon Den, you can walk up to the Pokémon Den and interact with it to start a Max Raid Battle with a Dynamax Pokémon. This event does not affect the Pokémon encountered within Dynamax Adventures.
Be sure to read the official Pokémon Sword and Pokémon Shield Max Raid Battle Tips article for advice on how to topple these opponents. Don’t wait, though—you don’t want to miss out on the chance to face this powerful Pokémon!