Can be done on the first mission after the tank encounter. Simply kill the rapelling enemies (4-5) then quickly run around the corner and toss a threat grenade with
. Then throw as many smart grenades as you can with
. You should hit 4 enemies that are running up the narrow staircase.
If this doesn't work for you, the video illustrates a point further in the lvl where it may be easier!
Hope this helps!