Although this will normally come as part of the story progression, it is actually possible to miss it there. During the quest "Rain of Arrows" in the Shelob quest-line you will first take out some archers and then you will begin the fight against the Warchief. If you take down most of the Warchief's health and then let a follower get the killing blow, you will actually not unlock the trophy.
There is always Warchief's in the army that you can go for, so it isn't a big issue if you do not get the kill and trophy here. Just something to watch out for if you are trying to get the trophy during this quest.