In Ostargar, there is a mission called "Tainted Blood" that takes place immediately before you become a Grey Warden. Collect the three Vials of Darkspawn Blood but not the Grey Warden Treaties. Return to camp, talk to Duncan, and report "We have the blood, but not the scrolls." This will exit out of the conversation, but will give you the experience for the mission. Repeat those steps until you are at the desired level. It takes very little time to reach the maximum of level 25 skip the conversations and speed up the process.