The ghost key just follows you around. You just need it to finish the stage.I think that ghost key thingy goes in there for you?
Otherwise i have never played that.
How do i get in there?
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), throw him through the gap filled with the two brown bricks at the right, then quickly descent to pick him up again before he falls in the water (if so, then quit the room and re-enter), and throw him a second time through the lower brick-filled gap. Climb up again, assume the crouching position, and let Wario drop while facing the wall and still crouching. He will creep into the gap where you can pick up the diamond.