Go to Pinna Park under a normal level where you can go around the place (like the Yoshi-go-Round level). Go into the main area and the pool where you fought Mecha-Bowser. Then turn right. There should be an area with platforms that turn when you stomp them or fire water at them. Find the two horizontal ones. Position yourself near the back of them and make the platform spin you into the wall. Tons of stars will fly out and you will be wedged in the wall! It is easy to get out.
Submitted by: Ian