The Nutcracker - Sweet!
Incapacitate an enemy, then shoot them in the testicles.
This cannot be gained on Cadet difficulty since the enemies die too easily. Luckily, you can go for the mission select screen and change the difficulty. Now, find a place to get a bunch of enemies to run toward you and mow them down (not headshot) as they come at you. If one falls and starts to bleed out, run up to him and shoot him squa' in deh nuts.