You need 180 gems to unlock this level.
The Toad Brigade is scattered all over Bullet Bill Base. You'll want to grab as many coins as you can. Walk around and spin the gear. One of the staircases has a pile of coins in it. Walk up the stairs, dodge the Bullet Bills, then aim the Bill Blasters at the nearby bricks. Let them fire once to break them, then spin them so they're aimed at the bricks on the staircase. While they break those bricks, grab Blue Toad. Spin the gear again so the Bullet Bills are aimed at the Flaptors flying near that same staircase. Once you defeat the Flaptors, aim the Bullet Bills away from the base. Walk down the stairs and spin the gear so you can walk up the other set of stairs. Drop down and you'll find Green Toad. Touch the item blocks to get the coins and Mushroom. Walk down the stairs and carefully pass the Bill Blaster to reach the gear. Spin it to aim it at the bricks on the platform moving in the lava. Once the bricks have been destroyed, cross the lava using the platform to reach Yellow Toad. Return to the gear and spin it so you can go through the archway in the staircase. Go through the staircase to another gear. Spin it to find a Gold Mushroom and the Star. Grab all of the coins on your way to get the Star.
Special Challenge: Collect 100 coins.