You have to play (and win!) all 3 maps in multiplayer.
Go to Customer-Match -> "Rome Prevails", "Hades and Back" and "XIV Legion". You can invite a friend or search for a random. The maps have some cooperative-stuff to do, where both player have to act. (not too hard)
After a match one player is able to select the next one.
Have fun and happy hunting, with an positive vote you can make my day