From the Super Mario Wiki
Goombas Galore is the secret level of Hoohoo Mountain 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 has two battles. The first battle starts off with only six Goombas, and a Goomba captain. The second and final battle has six Goomba Towers with a single Gold Beanie, and a Goomba Tower captain.