Ugh, so much for hoping for a reasonable way to do this one. Sounds like you need to hold on to Atlas Park, Kings Row and Skyway mayhem missions til you are level 30 or so, set for an 8 man team and rack up time and kills as fast as possible, then reset the mission by exiting the game and loading back in real fast. Sadly too late to do that for this dominator I am working on.
Second question, if you are on a team do all the enemy kills count for your defeat numbers or only the ones you personally kill?