To defeat the Witch King at the Pelennor Fields, make sure that you have Hadhod's Second Age Dwarven Mithril Piercing Axe and the Dwarven Mithril Helm. When you face the Witch King, use these attacks: Berethor's Stunning Strike and Ectehlion Wrath, Hadhod's Mountain Rage, and Eaoden's Enraging Blow and Rohirrim Rage. The Witch King has 200,562 HP and 59,098 AP.
Enter the fight with Perfect mode. Have your first character use the Banner of Elendil special. It will not actually stun the Witchking to make him stop attacking, but will disable his ability to counterattack for a great while. Then, have your next character use Morgul Decay, an item that completely rids the enemy of its armor. Now that the Witch King is completely vulnerable and will not counterattack, have your characters deal out their combos, such as Berethor's Ecthelion Wrath and the Rohirrim's Wrath of Edoras.