Age of Empires Online

To do this, start a co-op game through the quest screen, invite someone to your group (friend, stranger, whoever) via either the chat box or your friends list and then simply complete the level. Note that only the HOST will get credit for this.

Also note that you will get this as long as you host one of the co-op only quests required for achievements, such as the chain of co-op quests for "You Complete Me" and the uber hard co-op recommended "The Elite Meet in Crete". But, in general, it's often quicker to finish a level with a friend if you have one on hand, otherwise it may take longer than the quest takes to just find a partner.