This is a plot progression trophy and cannot be missed. You will earn it at the end of the DLC.
The achievement description is misleading. While it says that you will earn it after killing the last Grey Warden, you need to finish the entire fight (killing any non-Wardens who are left too), and watch the final cutscene before it will pop.
No special tactics or strategies are needed in the final fight. If you're having trouble with the combat anywhere, simply reduce the difficulty to Casual.