Near the end of the level HUNTER, you'll meet some allies and you will get attacked by some enemies in the vents. Make sure you pick up the revolver from the red locker, sit in the middle of the room, and pick off enemies as they come up to the vents.
There are 4 total vents, and each will spawn about 3 total enemies, for a total of 12 enemies which usually take 1 bullet to kill. If at any point a vent is opened, just pause and "Load Last Checkpoint", to try again.
After successfully defending the area without allowing any enemies to enter the room through the vents, achievement will unlock.
*Posting for Maka91, because he is only playing the digital version.