You can earn bonus points by eliminating multiple applians with the same shot. Take your time to line up your shots because of the slow speed at which the applians move. Applians drop down in waves. By waiting a few seconds, you can have them stacking on top of each other to set up large chains that can be eliminated by a single shot. When applians are in close proximity to the shield, a red exclamation mark will appear. Shoot them down during this time to earn "danger" bonus points. You will earn bonus points at the end of a level based ib how much shield power is remaining. Try not to get hit at all to maximize those points.