Ah, of course! The Xbox360 patch hasn't been released yet ... It has been completed and sent off to Microsoft but we are still awaiting a release for it. We have not been given a time frame for it's release so we can only say it should be out "imminently." Once the patch is released (along with the new DLC) you should only have to update and play a single battle to receive the achievement.
That was from an email I had received from them about a month or so ago. So HOPEFULLY we'll see it on the Game Marketplace soon.