In a two-player game, enter World 4's final castle. Make sure that at least one player has Ice power (either the flower or the penguin suit will suffice). Work your way through the level until you reach the first panel that you can pound in order to switch to the back of the grate. Have one player (the one with Ice power) stand on the rightmost "?" block just to the left of the panel. The other player should climb the grate to reach the switchable part, carefully avoiding the Koopa while climbing. When the Koopa is on the back of the panel, have the player with Ice power freeze it, and have the second player slam the panel around. The Koopa should now start spewing an endless stream of coins.
