In the Mushroom Forest, go to the background and read the sign next to the big spikes to activate the event.
There are two Bonsugees floating around in the area. Leap on one of them to make it freeze in place for a while. Use this opportunity to throw the other Bonsugee into the first one when they are close to each other.
When both have been knocked into each other three times, they will be defeated and the event is cleared.