After beating the record on the 12 combat scenarios, Admiral Ahern will invite you to participate in a final scenario. Accept his offer and the scenario will begin. As soon as you start, head towards the middle of the area and retrieve the data from the large object. Then, head to the stairs where you started and activate the turret. Now, you'll need to hold out for five minutes. Make use of your teammates and the biotic powers, they'll come in very handy. Once the five minutes is up, the achievement will unlock.