After picking up the shield item press X to make them spin around you, now you can press X again to fire them. Make sure you hit 3 different opponents with 1 set of shields, so that is 3 shots for 3 opponents, make them count !
If you are having trouble with this you can always plug in an extra 3 controllers and go into battle mode, pick up the shields with your main accounts controller and hit the other 3, easy does it. But really, you shouldn't have any trouble unlocking this through regular play.