In any of the shooting galleries, you will have one or two opportunities to get a random power up (power up are pretty obvious and are attached to a floating balloon across the screen). The Slow Motion power up (which looks like the achievement icon) slows down time and all the targets on the screen, making it easy to hit the moving targets, but makes new targets appear slower. To have better luck in hitting 20 targets under the slow-mo power up, try and make sure you have at least 20 or more targets on the screen. Once you have control of the power up, be sure to hit as many targets as quickly as possible. The power ups last around 10 seconds or so, there is not noise or timer or counter, letting you know when your power up will end and how many targets you've hit. Some levels are easier to get this in than others. Playing any level in Space Frontier or Balloon Garden will give you a good chance. Hold down to rapid-fire... in slow motion.