Arcade Shooting Gallery Achievement
An easy way to get this is to load up the first level on Montana (Woodlands) and as soon as you see a red sign in front of you look in to the distance on the right. Two mountain lions will run across the screen, shoot them and then don't shoot anything else. You will fail that section so will have to replay, rinse and repeat five times until you unlock the achievement. Also remember after each two kills to shoot the ground in order to lose your bonus or else you will get more points that required so will have to continue.