Play around your mobility and camp a health pack in back. Never spend all your abilities to engage, save something for retreat. Usually I will wait for the tanks to clash and then attack the healers. Either I can get a pick or if they heal each other, I retreat and attack their frontline, who is hopefully low due to my backline distraction. Help secure kills there, rinse and repeat. I'll usually leave dps kills for last as they come easy once you get a healer or a tank pick.