From the Super Mario Wiki
Mushroom Mayhem is the first floor in Mario vs. Donkey Kong 2: March of the Minis. The only enemies in the world are Shy Guys, Piranha Plants, and Circus Kongs. In the background of the levels, a rendering of Mushroom Castle can be seen. Later in the game, Toadstool Castle appears in the sixth floor. The music played in the various levels of Mushroom Mayhem are remixes of the Mini Mario levels from Mario vs. Donkey Kong. Most of the levels consist of girders, grass blocks for floors, and occasionally Gold Mini Mario.
During the battle against Donkey Kong, he hides behind a wall with three windows. Occasionally, he comes out, and the player has to shoot him with a mini. As for him, he attacks by throwing barrels and dumbbells which can be avoided by moving the cannon. After enough hits, he flees to Tropical Island.
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