I got this achievement at the same time I got the "My home — my rules" achievement (Liberate Mills Island). I don't know if it was bugged or not. You're supposed to kill all the pirates on 3 islands to liberate them. I figured I was supposed to travel around and liberate 3 separate islands but after liberating Mills Island this achievement was completed. Mills Island is a group of smaller islands. I'm not sure if those smaller islands were supposed to count or not. I would just follow the story line and liberate mills as its one of the first tasks assigned to you, or if you don't get the achievement liberate some of the other islands and you should get this achievement.