King of the Boos
From the Super Mario Wiki
King of the Boos is the second secret level of Chucklehuck Woods in the Minion Quest: The Search for Bowser mode in Mario & Luigi: Superstar Saga + Bowser's Minions. In order to obtain this level, the player has to use certain amiibo to unlock it.
The level consists of mostly Boos and Big Boos and has only two battles. The first battle has five Boos, two Big Boos, and a Big Boo captain. The second and final battle has three Big Boos in the front, with four Boos in the back, and a Big Boo captain.