EDIT: DUE TO SERVER CLOSURE ON 1-28-2014, THIS TROPHY IS NO LONGER UNLOCKABLE.
FYI, you do NOT need to do this with someone else. This is good news for those having trouble finding a boosting partner with the 1-28-14 deadline looming large.
Go to online, then cooperative. Choose to create a match and set up a mission and make it easy with the fewest enemies. Start the match. Once in the lobby, set up your soldier, then hit start to Deploy. I chose to eliminate enemy officers in the fishing village. I had to fight four groups of enemies with resupplies in between. There was one officer in each of the last two groups. Once the second officer was dead the mission ended even though I was still in a firefight with the enemy. Trophy popped at the mission stats screen.