From the Super Mario Wiki
First Battle! is the first level of the Bowser Jr.'s Journey mode of Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. This level cannot be replayed after beating it.
After Bowser departs to attend the emergency meeting at Peach's Castle, Roy and Wendy are speaking with the Best Fitness Friends about curing the Blorbs, while Bowser Jr. spars with Captain Goomba and three other Goombas under the condition that Bowser Jr. wins 10 matches, stating he needs to win six more. Afterwards, Bowser Jr. constantly increases the amount of wins he wants to seven and then to eight due to not being good at math.