You need to hang 5 enemies with rope darts for this achievement. Rope darts are a type of tool you can select within the [RB] menu. To hang an enemy, you must be on a tree branch. Hold [Y] and the dart will grab the enemy. Keep holding, and eventually you will drag him up in the air. Now move your [LS] and you will fall and he will raise up, hanging. This will use one of your darts out of your inventory. You can reload checkpoints if you are within a mission and found a good spot.