First, you'll need to level up enough to unlock the shockwave power-up. Now in game just grab the shockwave power-up and hold on to it. We still need to have one of the other players pick up the fireball power-up. Once they have done so allow yourself to get in front of them and as soon as they use the fireball, press
to release the shockwave. If the fireball is close enough to the blast it will deflect the fireball away and destroy it.