This achievement is called "Conqueror" to obtain this achievement you must defeat Knight Commander Meredith at the very end of the game. On normal difficulty it's a pretty straightforward fight. Just run up and attack her and your other companions will also aid you. Once you get her down to a certain point in health she will summon a large statue to fight you. Cullen will join the fight though he does very little, once you have defeated the statue Meredith will return to battle and it's best to use your most powerful skills against her.
Again at a certain point in health she will summon another statue, rinse and repeat on it. She will also summon some small statues but they aren't much of a problem. After while she will return to battle even with statues still on the field. Attack her and let your companions worry about the smaller statues. Keep at this until she is defeated, once she is the achievement will pop.