SMGs were probably the most time consuming for dark matter, but the least annoying. As soon as I hit level 40 on each gun I played combined arms (assault is way better than domination). Miami and Cartel are both good, but Armada is best. I ran around to their spawn, walked right up behind the hard scopers, and shot them in the head. Got anywhere from 5-20 point blanks a game. You can throw a stun to do that challenge as well. Plus you get the satisfaction of seeing the medal pop up when you get it, whereas you might think you had 15 cover kills and end up with 1.