This is a multiplayer achievement and can only be done in ranked matches.
While in cover, you have to press the RT to blind fire. The easiest way to do this is to play Golden Gun and get the Golden Gun. Take cover against a wall, preferably where enemies can’t come in behind you, and aim at the ground. When an enemy is where you are aiming, shoot. The Golden Gun’s bullets explode on impact so you will kill the enemy if he is above where you shot.
If you need help with this achievement, please use the Quantum of Solace Achievement Trading Thread.