Super Mario 64 DS/Goomboss Battle
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 2 stars you can obtain in this level: one from a star switch, and one from finding the 8 red coins.