Completed in 'Road to Victory' stage. There is some dialogue about taking a man's life, then you enter a room in slow motion. Kill everyone in 10 seconds.
Found it easiest to do with pistols and simply shooting everyone. Don't both with grenades, they take too long. If you do it before the slow motion stops, you'll get it.
If you don't get it, simply Restart from Last Checkpoint (which is right outside the room).