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After obtaining 8 stars, you will be able to open the door in the back of the Rec Room. There is a picture of Mario in the room behind it. Going inside will get you into the stage.

To reach Goomboss, you will need to reach the red X on the map.

To defeat Goomboss, you will need to run around behind him, then eat a Goomba that is running behind him. The, you can either spit it out or make an egg to hit Goomboss. It will take 3 hits to defeat him, but he gets faster with every hit. Upon defeating him, you will get the key to Mario's room, unlocking him.

There are two stars you can obtain in this level. There is a star switch and 8 red coins.