In Ostargar, there is a mission called Tainted Blood. Simply collect the three Vials of Darkspawn Blood but NOT the Grey Warden Treaties. Return to camp and talk to Duncan and report "We have the blood, but not the scrolls." This will exit out of the conversation, but WILL get the experience for the mission. Rinse and repeat until you are at the desired level. It takes very little time to shoot up to lvl 25(cap) if you press X to skip the conversations and speed up the process.