You don't need to leave the area or kill all the mobs, but it's something to do while you wait. Well, except the leaving the area part. Don't do that. Stay there and keep your eyes open. It's possible you missed him on one of your trips back and forth, especially if you are on a populated server. Also, remember to look up. He's a rare and will spawn on his own eventually. I don't know his spawn timer, but it could be several hours or more. Older rares are not like the new "rares". Just be patient, and best of luck to you. You'll get him!
