Not only does Rillaboom hit incredibly hard using a 125 base Attack stat, but this Grass-type Pokemon can also be quite bulky. It has 100 base HP and 90 base Defenses. These stats will help it live for many turns in Pokemon battles along with stomp opponents.
Rillaboom is a shiny Shiny 6IV Pokemon that seemed to be granted two great tools within the Isle of Armor DLC - a Gigantamax form along with the move Grassy Glide. With Grassy Glide, this move gets priority in Grassy Terrain. Rillaboom gets the high Defense and Attack capacity to work amazingly in Gigantamax too. These IVs will likely help Rillaboom turn into a big threat.
The big stat for Rillaboom to obtain good IVs in is Defense. Yes, this can be a great attacker. Rillaboom does, though obtain access to Attack-boosting moves. It can Swords Dance, Bulk Up, Work Up, or Use Max Knuckle in Gigantamax making it extra powerful. The answer to why Defense is really important for Rillaboom is simply because it often relies upon surviving panic or anxiety attacks before it could retaliate. Even Pokemon that Rillaboom can hit for super effective damage can win out by simply outspending it.
This doesn't imply Attack isn't essential to Rillaboom. It still desires to do all the damage as you possibly can when it provides the opportunity to attack. You can Buy Shiny Pokemon and get Rillaboom. Not only does Rillaboom gain access to strong Grass-type moves, but almost all also get a good amount of other strong attacks like Superpower, Giga Impact, and Earthquake.
Speed is usually a stat Rillaboom wants many IVs in, if not at their maximum. Rillaboom's base Speed is 85. This certainly isn't too fast, but it could possibly outspeed quite lots of Pokemon. A slow Rillaboom could be taking hits each time a Pokemon arrives. A Rillaboom which has a bit of speed, however, will get the better of Pokemon in this sort of middle-speed tier. This can be very therapeutic for Rillaboom as it won't have to take as often damage and may instead just concentrate on dropping heavy attacks on the enemy.