The easiest way to get this to appear is to change your multiplayer preferences so only Secure Area game modes come up on the map Clubhouse. Then the rest is basically up to you and your team to win it.
The devs have changed the requirements of this achievement to simply win 100 defense rounds in multiplayer. Easiest thing to do is take things slow, stick close to the objective and don't rush towards attackers. There's no quick way to get this, as you would need at least 30-50 games if you win every single defense round you've ever played.