When you need to destroy the 4 crystals in the level ''Minas Morgul'' of the War of the Ring campaign you can choose to be Aragorn (this is the second chance to be Aragorn, the first one is when entering the castle) and need to defeat 3 Nazgul. Not that hard because there are plenty of them and they arent that hard to defeat.
When fighting the Nazguls, use combos to take huge chunks of their life at once. There are plenty of Mages around, so once your health goes low, run to one and wait to be healed. It's recommended that you heal once your health reaches the 60% mark, since the Nazguls will combo you, and hit you when you're still on the ground, so you'll need as much health as possible. There's no time limit for this one, so take your time, heal as often as you can, and you should be fine.
Note that your AI bots will kill Nazguls as well, and those will not count towards your total.