In the Wireless multiplayer (mario vs luigi) do the grassy level and where there are two stairs and a pitfall this is where the glitch will take place. first get a big mushroom and wreck everything and the koopa to the left of the left stairs will walk further on seeing as the stairs are gone. so when the koopa is right in the middle of where the left stairs would be get a star coin quick and everything will be back in place. so the koopa will now be stuck in the stairs.
If this isnt enough of an explanation tell me please.