To encounter Shellder in a Max Raid Battle, you'll need to connect your Nintendo Switch system to the internet through the 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 in the Wild Area, you can walk up to the Pokémon Den and interact with it to start a Max Raid Battle. This event does not affect the Pokémon encountered within Dynamax Adventures.
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Catch Shellder in Max Raid Battles
Crack open your Pokémon Sword and Pokémon Shield game for some Bivalve Pokémon fun, and you might encounter a Shiny Shellder.www.pokemon.com