This achievement requires that your entire party be level 16. You then turn on Even Ground and kill the Fereldan Frostback in the Hinterlands (north of the northeastern campsite). If, like me, your main problem is finding a save with a decent level that hasn't killed the dragon, this video may help you. I could only find one at level 13 and I had no interest in doing more quests and shit after I've already done so much. I used this method to level to 16.
THIS IS NOT MY VIDEO. I just wanted to share it because it seems like the best way to level. I know there's a method involving giants but they eventually stop respawning and if you're underleveled you aren't killing them quickly anyway. Basically you unlock the Hissing Wastes and bring a mage, go to the camp northwest of the starting camp (closest one) and then go into some ruins right by the camp, down the stairs and through the door. There's 4 torches here and you're supposed to hit them in a specific order. Instead, we want to use the torch in the back, on the right side (assuming you're facing inside from the entrance). When you use the torch, 2 rage demons and 2 of the smaller demons spawn. Kill them. After they're dead, the torch can be lit again and they'll respawn. It's obviously tedious, but it gets the job done. The Demons are much easier to kill than a giant, so I was futzing around the computer and letting my allies kill them. And, when you need to restock potions, the camp is like 100 feet away. You'll get about 400 experience per group.
Another thing to note is that near the ruins (above ground) is some tables with papers on them. One of the papers gave me 5000 experience, so make sure to snag that.