Help a Captain survive a Recruitment Power Struggle, and then kill him and all his new recruits.

When you look around the map, you'll notice there are Red boxes that indicate Uruk Captain events. Look for one that says Recruitment and start the mission. The captain will try recruiting new members, bu the orcs will try to kill him. So do your best to kill all the orcs around him while avoiding that captain. If you manage to kill the enemies and the captain is following you, break his line of sight (His indicator will be yellow if he can't see you) and he'll stop looking for you. Eventually, the mission will be complete and he'll gain a power level. Now kill him and any orcs around him as soon as the mission is over and the achievement is yours.