The Bristleback is east of the Lost City. Go out of the city that way, up all of those steps and ramps and most steps, and yea you get the picture. Then when you get to the top go right. If you see a Legendary Bristleback coming at you then you found the right place. Just go carefully in there, hug the wall and go to the left. If you just keep hugging the wall you can avoid the mobs.