Killing a single target in a hoard? Try luring it out. Try sniping it from afar. Try jumping down on it. Try sneaking up on it. Stealth kill the other enemies. Go in swords blazing. There are many ways to take down a single target. And if we are talking about Captains, they have a lot of weaknesses most of the time. I'm not far into the story myself, only finished the first mission in the overworld, but I am walking around the map just killing Uruk's and collecting stuff.