Its best to have a captain that is already dominated and under your control and have the dispatch ability unlocked.
Just check the map for power struggles labeled "Recruitment". If none appear at first, use the towers to fast forward time until one shows up.
Once in the Recruitment power struggle follow your captain to each recruitment area. Use the bow to headshot the yellow labeled uruks. Once the recruitment is over press down on the d-pad to kill all branded uruks.
Best to leave this till last as the dispatch ability is on the last tier of abilities.