This achievement is awarded on completion of the 3rd level in episode 1, the Labyrinth of Death.
As ever, the best tactic is always to put as much distance between you and the zombies, and snipe away. Thankfully, the enclosed spaces in the underground tunnels help to funnel the zombies in a lot of this level, but always watch out for skeletons that tend to flank. I find the best way to dispatch them is via a kick rather than shoot them (LB).
There are two main difficult areas I found - the first was with the mounted machine guns and the hoard. Before you start, landmine both staircases, as this will buy you more time to spend on the gun. Target the suicide bombers as a priority and just fire constantly, as ammo is infinite. If/when you get overwhelmed, back into the room, as use the doorway as a choke point for trip mines, and shotgun.
The other main difficult bit is the very last standoff, which sees a large wave of zombies, coupled with a heavy with an MG42 (this then repeats). The knack I found here was to go up on one of the platforms, and snipe away at the regular zombies, avoiding the MG42 guy. When he gets close/starts coming up the stairs, vault over the platform, and sprint to the other one, the other side. This way you can thin out all the regular zombies, and can then concentrate on the heavy from distance. If he gets close to the other platform, just rinse and repeat. Good luck!