From the Super Mario Wiki
Showdown Time!! is the ninth level of Bowser's Castle in the Minion Quest: The Search for Bowser mode in Mario & Luigi: Superstar Saga + Bowser's Minions, and by extension, the penultimate level of the game.
The four captains, the Elite Trio, and the Koopalings finally reach Fawful, who makes a failed attempt to brainwash everyone in one go. Even with Fawful's confidence, Captain Goomba refuses to give in, and the battle ensues. They battle Elite Limbo Bros, Parabeanies, Beanerang Bros, Rexes, Elite Troopeas, Scaratroopeas, Piranha Beans, Lakipeas, Fuzzbushes, Clumphs, and Fawful himself. Fawful is seemingly defeated, and Captain Goomba reunites with what seems to be Bowser. "Bowser", however, tells Captain Goomba to "destroy" Mario, surprising him.