This achievement is obtained on the level "The Road to Victory".
You come up to a breach section that is a direct nod to the Call of Duty series. When you get there you will bust the doors open and slow motion will kick in. Simply move forward killing everybody in the room. The doors in front will bust open and take them out as well. You can use grenades but I found the enemies dodge them and can make it harder to shoot them.Time is slowed down as well actually giving you more like 15 seconds than 10 seconds to complete this section.