An excellent mission for this is the air superiority mission in Moscow; at the very end there is a fairly long DFM against Markov. Get in a three person co-op game, and then get to this part of the mission - it's scripted to last a while, giving you all plenty of time to stack up into the DFM mode together and then take him out. I don't think I fired the killshot and I got the achievement, so it may be as simple as being in DFM when someone else does all the work.