The Frostback Dragon is located in the Hinterlands, normally this dragon will be a level 12 so you will have likely killed it too early. If you're fortunate you can load up a previous save to when the dragon is still alive and attempt to get this trophy, otherwise you will have to start a new game and get it. One good thing is that the trial can be turned on once you have reached level 16 and does not need to be turned on for the entire play-through. Also, just to be safe, be sure your entire party is also level 16.