Goomboss Battle is the first boss level of Super Mario 64 DS. Upon completing it, Yoshi will earn the key to unlock Mario.
The level is set in a gloomy forest on a series of platforms above a field of toxic gas, which should simply be avoided. Navigating the level shouldn't be too troublesome, despite its vastness and appearance. The goal here is to find and enter Goomboss' chamber, where Yoshi must defeat him.
There are two Power Stars in this level; One for finding eight Red Coins, and a Switch Star. However, because Mario's wall kick ability is needed for the Switch Star, and because Luigi and Wario are needed to collect certain Red Coins, the player will need to make at least one return trip to this level to collect those Stars.
The entrance to the level is the Mario painting, which is behind the 8-Star door in the Rec Room.