make sure you pick up the silver balls during battle. you will find lots more of these during the bonus waves. at the shop there will be an icon at the bottom to use the pachinko machine. just swipe the handle, drag the mouse, or press the down arrow or space bar to launch them. it's best to wait until you have more than just a few at a time since your multiplier goes up depending on how many you have in play at once. as soon as you launch your 1000th ball, the achievement will pop.