Halo Wars 2

This achievement must be done in an online game with other players, whether it is through a private match or online game.

Before a match of Firefight Terminus begins, you and your teammates will have some time to choose your desired Leader. If you're playing with a friend, simply coordinate so that one player is a UNSC Leader (Anders, Cutter, Isabel, etc.) and the other is playing as a Banished Leader (Atriox, Decimus, Shipmaster, etc.). On the other hand, if you are playing online with random teammates, wait for them to pick their Leader before choosing your own (unless you have a mic - then you can easily communicate).

Once you have completed wave 20 where one player is a UNSC Leader and another a Banished Leader, this achievement will unlock.

Refer below to "Fortress" for more information on reaching/defeating wave 20.