For this you'll need to use Wraith Flash by hitting
+
when you combo meter is red. Keep in mind that a higher combo meter will result in a flash with a larger radius and an increased chance that an uruk will be branded. When you do successfully brand an uruk with Wraith Flash you'll get one more tally towards your 50 needed. As Wraith Flash is a very effective means of crowd control you'll get this quite easily during your run through the dlc's story campaign.