Tap and hold the screen to constantly charge up your shots. If done correctly, you will be able to shoot a fully charged shot at every wave. Use a second finger to curve the shot, then let release it, while keeping the other tap held. You can create a curved shot by moving a second finger in the opposite direction of your first finger to make the shot move in a U-turn and return to Earth. Use this type of shot to get big chains in separate sets of applians.