如果你买了DLC的话 很好取得 我是在打DLC的时候取得的  没有买也没有关系 看下面

The animals that can kill enemies are: Bears, bobcats, cougars, and wolves. It has been reported that a fox or an elk can also kill an enemy for you to gain the achievement. 

There are scripted events in the Frontier area where one of these animals kills an enemy. They have somewhat fixed locations. Go to the Packanack hunting zone on the western part of the map for the best luck. Try to find the restricted area (highlighted in red). Go up a hill to the west and you will encounter several bobcats, as well as two hunters. You may not be in open conflict with any of the hunters or animals or they will not attack each other. If this happens, run away. Sometimes a group of enemies are walking on a trail and you can attack them and run to an area high in "predator" animals. Run off and then circle back to follow them and you could have a chance to see them come in contact with an animal

http://www.youtube.com/watch?v=pSQmxzLAfy8