For this achievement you must defeat Terranova who is the last of the new hunters in the DLC. Much like the base game where Bounty Hunters come to kill you much like the five stars in GTA, you must reach Rank 5 and only Rank 5 for a Bounty Hunter to show up by killing humans and destroying their boats then kill them once they show up. You can easily knock off the first four by starting the DLC, but for Terranova herself she won't appear till you have to progress further in the DLC where upon playing one of the main objectives tells you to reach Rank 5, after that its just kill her and the achievement will pop.