You need the Owl: Attack Skill for this achievement. You can reduce the cooldown to 40 sec but it is not necessary in my opinion. The Owl: Weapon Drop Skill does not count! Only Owl: Attack counts. You can do this everywhere on the map, best places are outposts or bonfires. Use your owl, identify a target and use RT when the game tells you (Owl: Attack). Repeat this 15 times during your playthrough and the achievement is yours.
Animals counts also as enemy (thanks to B0ngAAA in the comments)