Shoot an enemy in the foot with the Welrod pistol (or another weak pistol/rifle). Sometimes this will make them fall down. Then, move close to them, take out the sniper rifle, and shoot between their legs to get "The Nutcracker - Sweet!" achievement. One area this can easily be done is in Mission 8: Allagra Fortress. The enemies in this mission have more health and do not die so quickly by your pistol shots, giving you more attempts to incapacitate them. You can do this with the first two enemies in the mission. After incapacitating the enemy, you may want to create a manual save game -- just in case the testicle shot goes wrong you can reload and try again.