Tune your aim. Your objective doesn't stop of moving around and your score depends on the number of accurate impacts and the % of effectiveness. You will not be able to waste a single bullet.
Second part of the sniper hero, but a little bit harder. Try your aim and reflexes! Use the mouse to aim and shoot, the space bar to activate or deactivate the zoom, R key to recharge, A key to duck and the S key to get up.