Not really a solution, but something to complete all those mentioned in Zeifer95 excellent solution and comments, where I also added it as comment:
"Guys I found a solution, that really worked, in Reddit. The thing is that you have to loot the skeleton in the centre of Kradia isle. The moment I did it, the achievement popped."
As pointed out in comments, Kradia Island is to the south of Samos, a really small one.