When you are against the witch king at Pelenor Fields the easiest way to defeat it is by having Berethor, Hadhod (Make sure to have his 2nd Age Dwarven Mithril Piercing Axe and The Dwarven Mithril Helm) and Eadon. With these three, use these attacks: Berethor's Stunning Strike and Ecthelion Wrath. Hadhod's Mountain Rage and Eadon's Enraging Blow and Rohirim Rage. You will beat him in no time.