From the Super Mario Wiki
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This level has several Polterguys the player needs to solidify by pressing their respective Color Switches and using Handstand Double Jumps or Backflips to jump between them. To get to the Key, the player has to line up the yellow Polterguys to create staired platforms toward the left, then the player has to jump onto them using a platform below and while avoiding the other Polterguys. Once the Key is grabbed, the player simply needs to return to the ground while avoiding the Polterguys again.