Easy to get this when you unlock the 'More Ways to play' on the start screen. You unlock it at about level 3-2 if I remember correctly. When you do unlock it, press the 'Wall-nut Bowling' mini game. I got it quite quickly, though to get it easily, play properly then about 2 - 3 mins in, let them get close to the red line then throw the Wall-nut. when there is a load on the screen. Shouldn't be too difficult.