From the Super Mario Wiki
Boomerang Rally is a level in Bowser's Castle (West) in the Bowser Jr.'s Journey mode of Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey.
The level consists of Fawful Guys, Bob-ombs, Boomerang Bros, and White Magikoopas. There are a total of four battles. The first battle is made up of Fawful Guys and Boomerang Bros. The second and third battles are made up of Fawful Guys, Bob-ombs, and Boomerang Bros. The fourth and final battle has Boomerang Bros and Fawful Guys as enemy troops, and a White Magikoopa inflicted with Malatone Formula:X as the captain.