This solution may very much be YMMV, however the game basically served this up for me on a plate and it was easy to do.
When you're battling the final Warchief, he will disappear and you'll leave the cave to find him outside. After Shadow Striking over to him you begin combat again. During this combat, lots of Ghuls show up and it's very easy to brand a bunch of these by using
when your combo is red. You'll probably struggle to even defeat the Warchief without doing this. Ideally you'll brand a Spitter Ghul or two during this.
Now I don't know if the following was scripted or not, and so you may not experience this, but as the fight continued, both Caragaths and a Graug ran into join the battle. At that point it was simply a case of Shadow Branding both of them, and achievement unlocked. You can Shadow Brand by
on both targets. I'd recommend doing the Caragath first, then the Graug, as you don't have to dismount first. If necessary, then just look for a Spitter Ghul if you still don't have one branded. The Graug should stay alive for a while, and you could ideally ride the Caragath round looking for a Ghul if you still need one.