Kill a Graug by dispatching your Ghûls on him.
What a pain. So, there is a mission where you first learn to wraith brand ghûls. At the end, Trovin will show you how they fair against a graug. Before they kill the graug, dispatch your ghûls by hitting down on the D-pad. They will explode, killing the graug and unlocking this achievement.
This can be done during the mission "Hypocritical Oat". If you didn't do it during, you still can, but it can be a massive thorn in your side to get it to work. Here is what I did...
Start at this save tower:
Jump down and look towards the stronghold and shoot down the hanging graug bait here to trigger a graug every time. He'll spawn near the base, but not actually inside it.
Next, I lured a few nearby orcs to the graug to work in some damage; we want the graug weak, but not dead. Once about half the health was gone from said graug, I triggered the first nest of ghûls, used an orc to build up my combo and mass branded as many ghûls as I could.
Shoot two arrows into the graug's face to stun it. Your ghûls will latch onto the beast and deal damage. With enough on them, hit down on your D-pad to explode all of the littler critters, killing the graug. You can kill it with as few as two if its health is low enough, but the more the better.