From the Super Mario Wiki
Explosive Troops is the first level in Bowser's Castle (West) in the Bowser Jr.'s Journey mode of Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey.
This level has four battles, with a Bomb Boo acting as captain and placed in the center for all of them; in the second and fourth rounds, this specific Bomb Boo is under the effects of Malatone Formula:X. The first three rounds all have Spinies in a square formation, with other Bomb Boos, Bob-ombs and Alarm Bob-ombs (who make their only appearance here) stationed at the formation's very top and bottom. For the formation in the fourth round, the Spinies and other enemies swap positions.