Chief Chilly Challenge
From the Super Mario Wiki
This level is set in a snow world, similar to Cool Cool Mountain or Snowman's Land. It is fairly linear. Upon reaching the end, Luigi must fight Chief Chilly. As with the other boss levels, there are two Power Stars that can be found here.
The entrance to this level is the Wario painting, which is in the mirror room on the second floor, although only Luigi can enter it. When Luigi enters the mirror room, Power Flowers will appear, which he can use to pass through to the other side of the mirror. On the other side of the mirror is the Wario painting.
The first Power Star requires for the player to collect eight Red Coins. However, while it is impossible to backtrack past certain points throughout the level, all the Red Coins can be obtained on the first playthrough provided the player doesn't miss any. Ultimately, this is the only boss level Star that can be obtained on the first playthrough.
This Star requires Metal Wario to reach, and must be obtained on a return visit. Near the beginning of the level is a ! Block containing a Power Flower that Luigi (wearing Wario's Cap) can use to become Metal Wario and safely cross a windy ledge. On the other side is a black Brick Block containing the Power Star.