This achievement is easy going if you do it in a 1v1 session.

If played on "urban assault". If you start on the creature side run up to the bridge, where the "newbie" stands, and kill him quickly. normally it takes 40-50 seconds to kill him. When the round changes and you are on the protector side, just run around the map till the timer is up, than you'll get the win.

In most cases, when you have won the previous match, you would started on the attacker side, so repeat the upper discription. But if you start as the protector, try to stay alive as long as possible (maybe the whole 3 mins), then when the round changes, you have enough time to kill the "newbie".

With this method you also could get the achivement "Protect and Serve"