Start a game of joust and let all other characters with the exception of the one you are controlling stay on the ground.
Now, just hop around the screen until you see a '?' block appear. Try to grab the block while traveling in the direction facing the players on the ground. If it is a laser, simply let yourself drop down and do your best to jump right as you are about to land. If done correctly (or you just get lucky enough) your player will fire off a laser close to the ground and hit one of the other players, which will unlock the achievement.
This is not necessarily difficult, but a bit frustrating. For one, the blocks don't always appear frequently. When one does appear, there is a chance you will get another item instead. If you do get the laser, it only lasts for a couple of seconds. And unless you time it just perfectly, you will either jump too far from the ground to shoot anybody or you will smash into the ground.
A little bit of practice and quite a bit of patience will make it work though.