For this trophy you need to scare a Warchief and then kill him. Here is what I would do in number order.
1. Choose a Warchief and gain intel on him until you learn what his fear is. Examples from what I have seen is Fire and Caragors.
2. Locate the Warchief and use his fear to scare him
3. After you scare him (you know that happens when a scene with the Warchief starts to run away and it will say Fear of ......). Catch him and finish him off and the trophy is yours.
I have attached a video it is not mine but Mr. Reign as a visual reference to what you need to do.