To get this trophy, which is called "Ghuls Gone Wild", you will have to kill a Graug by dispatching your army of Ghuls on him.
First off, you won't be able to earn the ability to brand Ghuls until you get to the mission that's called "Hypocritical Oath." In my opinion, that's the best mission to get this trophy. Sure, it's possible to find a spot or two that has both a Ghul mound and Graug Bait nearby. Problem is that there are random monsters, such as Uruks and Caragaths, that will appear and kill them quickly, so I don't recommend that method.
Instead, play through the "Hypocritical Oath" mission. That mission, by the way, is unlocked once you have destroyed at least four out of the five warchiefs.
Begin the mission by branding your army of ghuls and getting as many as you can. You have at least two or three times to accomplish this. At the end of the mission, a Wretched Graug appears and challenges you and your army of ghuls. If you have a lot of them by your side, then it's a piece of cake.
When the fight starts, immediately put two Charged Shots from your bow and arrow into the Graug's head in order to stun him. Then let the Ghuls go on his body and start damaging him as quick as they can. If you don't have enough of an army and you only whittle him down for a bit, chances are that he'll regain his footing, which means you'll have to use two more Charged Shots on the Graug's head to stun him once again.
Once he is at low health and the Ghuls are on his body again, IMMEDIATELY hit
on the directional pad and all of your Ghuls, including the ones that are on the beast, will explode and die. You do this correctly and the trophy unlocks.
For further information on how to do it correctly, I've attached a video guide with this solution I wrote. Credit goes to richard919 for the video. It's short, but it's to the point and shows you what you need to do at the very end for the trophy. Have fun and good luck!