Age of Empires Online

As you progress through the game and reach around lvl 25/halfway through the Argos quests, you will eventually be given a quest called "Live Together, Die Together" which is the start of a co-op only quest chain. Finish this chain of quests and the final quest, "You Complete Me", to grab this achievement.

Starting the maps off with the corner spawn points with cliffs on 2 sides of you making a narrow entrance to the area you start in is the best bet for each quest as they can be pretty difficult to start out in. To get the map of preference just keep restarting the mission.