How To Get Gigantamax Rillaboom?

In addition to the three New Legendary Pokemon and two New Galarian Forms found in the Isle of Armor Expansion Pass, there are also a total of seven brand New Gigantamax Forms. Two of these Gigantamax Forms belong to the Legendary Pokemon known as Urshifu, one for each of its two different regular forms. Unlike the other Pokemon with Gigantamax Forms, Urshifu can only obtain its Gigantamax Forms by way of a very special Max Soup formula. Two more belong to Venusaur and Blastoise, completing the Kanto Starters Trio in terms of Gigantamax Forms. As you progress through the Isle of Armor Story, you will receive one of these two with their Gigantamax Form as a Gift Pokemon.

The final three brand New Gigantamax Forms belong to each of The Galar Region Starter Pokemon, including Rillaboom, Cinderace, and Inteleon. When it was first announced that these three would also be obtaining Gigantamax Forms, it was believed that this would be done as a reward for only your Starter Pokemon chosen at the beginning of the game. However, we now know that isn't the case.

Instead, Rillaboom, Cinderace and Inteleon each have a different method rather than Max Raid Battles (like the other Gigantamax Forms) to obtaining their Gigantamax Forms. In order to get Gigantamax Rillaboom, you must create and give it Max Soup with the help of Mega Mushrooms. Since Rillaboom earns its Gigantamax Form via Max Soup, that means you can have any Rillaboom you have earn its Gigantamax Form. This is all the better as you can use the Max Soup on a Rillaboom with its Hidden Ability (which is the better of the two), rather than its Regular Ability like the Starter Pokemon chosen at the beginning of the game is locked in on having. It's a win/win!