Go to the end, just before the flag. Near the flag is a pipe facing to the west with Goombas walking out through it. Get a Koopa shell return to that pipe, then let go of the Koopa shell and make sure you throw it eastward. The Koopa shell will come back and forth hitting every Goomba that comes out that pipe, which will add points to every hit. Jump over the Koopa shell while it is going back and forth without touching it.