If the game has gotten to the point where the Nazgul are in the middle of a horde of Morgul Orcs with Overseers, then chances are you probably have the ability to get Rangers, and have a standing army of Tower and Citadel Guards with upgrades. It is extremely easy to single out and kill Nazgul, because they stick out like a sore thumb in the middle of Mordor's armies. Gondors heroes, with the exception of Gandalf the White, blend in with the troops extremely well, and can sneak up on, and kill, the Nazgul quite easily.