There are two ways to get this. One - play the game like you should, fight as many enemies as possible and do as many sidequests as possible. I got my level 20 towards the end of the game.
The other way to do it is to get to Ostagard and start the quest where you get three vials of darkspawn blood. Get those three vials but don't get the treaties. Return to Duncan and tell him you have the vials but not the treaties. You will receive experience. Now keep talking to him stating the same thing, you will continue receiving experience. Grind up to level 20 and enjoy achievement goodness.
Edit: For those of you not getting this, this does work. The game will not show you "completing" the quest over and over, but if you check your experience gauge after every time talking to Duncan and stating you have the blood but not the treaties, the experience gauge will go up. It takes approx 30-50 minutes to go from level 4 to level 20 (depending on how quickly you push buttons and if you skip the cutscenes). It just takes patience.