This is a multiplayer achievement and can only be done in ranked matches.
This should come naturally while playing multiplayer. To get in cover, press A near a wall or something you can hide behind. To shoot someone while in cover, you can either blind fire by just pressing RT or you can aim and shoot them by pressing LT to aim and then RT to shoot.
The easiest way to get kills while in cover is to play Golden Gun mode and get the Golden Gun. It will only take one shot to kill someone with the Golden Gun and everyone will be coming towards you to try and kill you to get the gun. Aim at the ground by their feet because the bullet will explode when it hits the ground and it will kill them.
If you need help with this achievement, please use the Quantum of Solace Achievement Trading Thread.