On your way to killing the 4th warchief in the DLC you will have to do a mission in which you learn how to bait mounds to force ghuls to come out. Once you have completed this mission you will be able to start baiting mounds at will. All you have to do to bait a mound is shoot an arrow at it. They are easy to spot while aiming with your bow. They are small mounds of dirt that have a little ghuls head icon above them. You must bait at least 3 during the training mission and another 3 during the next mission to draw out the warchief. The next mission has you baiting mounds to draw out ghuls to lean how to brand and control them so this achievement should be unmissable as long as you are following the campaign missions. If you do not have this by the time you finish the main story, just ride around open fields of grass looking for mounds to shoot, there are plenty.