During the mission "No Russian", after leaving the airport and getting to the main fight, there will be lines of riot police standing nice and close. Just lob a nade launcher round into one of them, and chances are it will kill the two men around him.
This can also be done on the level "Wolverines!" when defending the burger town. There will be a point in time where the person on the radio will tell you that there are 15 men approaching your position. You can try and get a grenade in the middle of them. (this is also a good time to get "10-plus foot mobiles" for killing 10 with one predator missile)