LingTone, I've had the same issue from 2 play though's. it's the Heron Skill your speaking of, and to clarify ... your objective is to kill someone above you, not below you. (forgive me if I misunderstood your post). There is a video tutorial when you select the objective icon in the Heron Skills. Even after I've captured all the outposts, I've not ever seen anyone on a climbable platform from which I could gain this skill. It's the only skill I've not gotten after playing though twice.
Considering the Video Tutorial show's pirates, I'll just assume there is at least one pirate base that will contain two pirates on a climbable platform from which you can achieve this skill. So the good news is, if you go ahead and finish the game, a future update will allow you to reset all the outposts, and thus you'll be able to find the base with the two pirates on the platform above a ladder, or climbing vine, and achieve the skill.