Be Leery of Lightning
From the Super Mario Wiki
Be Leery of Lightning is a post-game level in the Bowser Jr.'s Journey mode of Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey.
At the start of every round during this level, the enemy captain will use the Lightning Captain Command, shrinking all allies for the entire round. However, Wendy's Doppelganger command or Larry's Call for Backup command can turn the tide on this, as their clones/reinforcements will not be shrunk.