From the Super Mario Wiki
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The player first has to climb up the ladder on the left to activate the Mini-Mario, and then climb back down the ladder. Moving right and climbing up another ladder, and then moving to the left will cause the Mini-Mario to jump up two platforms, collecting two Presents along the way. The player then has to lead the Mini-Mario up a series of small platforms, avoiding the Tane Pakkuns' projectiles as they jump up. At the end of the second flight of platforms is the locked door.