On 1-3, wait for the koopa to be all the way to the left then hit him into his shell. Then hit him straight forward and into his shell again. You have to get him at the perfect distance between the wall and you on him if you're going to do this. Then stay still on him and he'll start moving, then he'll hit their wall and you can bounce on him again and again and again! You can get up to 500 1-Ups!