This achievement is during the level "Road to victory".
You cannot miss it as it is a Call of Duty piss take where you breach a room full of enemies.
When you breach switch to machine guns (you default to pistols) and move into the room, as the far doors open and more enemies will emerge you have to kill.
If you don't manage it in the slow mo time (or just after as I think you get 1-2 seconds plus) just restart the checkpoint as it's just before. It took me a good few tries and I tried to kill those on the left first, sweeping right, then hitting the guys coming in from the doors.