You have 100 seconds to clear this level, but it’s not a particularly long course and can be completed fast. Hit the Roulette Block at the beginning to try and get a good power-up, the enter the door to find yourself in a silhouette section. You should see a star right above you, which will require a little bit to get. For now, head to the right and use the cannonball to get onto the second level. Get to the top and use the cannonball to reach the platform with the Boomerang Flower. Now you can head left and use a boomerang to collect Green Star #1.
Continue right and right before the Goomba Tower will be Green Star #2, out in the open. The Goomba Tower is the key to getting out of this level, by the way, so just keep tossing boomerangs at it until you cut it down to a reasonable size. Defeat all of the Goombas to make a Warp Box appear, but before using it, wall jump to the platform in the upper right to collect Green Star #3. Take the Warp Box now, jump on the bricks, then jump again to discover a hidden block. Go back to the doors and use the left one, then you can get to the top of the flagpole.