How do you defeat the witch king?

It is very simple really, just get Eglost to use crippling shot to stop his attacks, then get Idril to either use gift of Galadirial or loudwater fury.

Use Berathors defended strike (I think but it is similar) just get Eaowen to use that 5 strike manouver!

The best combination IS Berethor-Idrial-Hadhod, since Eowyn doesn't do much good (her attacks are lame; her 5-strike attack [sorry, can't remember the name] added up together WITHOUT misses is only as good as Idrial's Loudwater Fury.

With Berethor, remember to use Defensive Strike as Angmar can Counter Attack everything. And with Eowyn, remember to have someone heal her with Kingsfoil or something of the sort [sorry, screwed memory] (or Idrial's Gift of Galadriel, since she has loads of AP and has nothing much to do with it :P).

With Idrial and Hadhod, use their Spirit Powers so the Angmar can't counter attack. Don't try their Sword or Axe Craft; they'll just miss.

If you use Elegost, use Crippling Shot, as if doesn't let Angmar Counter Attack. But watch it -- Elegost might miss sometimes.

For the Witch-King it is essential that you have Morgul Decay,but Berethor-Idrial-Hadhob is the best combination.Use Bererhor's Double Attack Ecthelion Wrath or Gondor Rampage if you have unlocked it.Also I suggest using Defensive strike(rarely) because the WK can't counter attack.Targeted strike can be very usefull too.Make Idrial cast Aura's Valar to revive any character that may die.Hadhod is usefull to cast Mountain Shield.It will grant protection for some hits.Eowyn should attack with her 5strike hit and make Idrial cast Haste of the Elves,since her Pasiive skills increase its effect.Doing the above will grant you protection,taking actions more often and a good damage.Remember to cast Gift of Galadriel when needed.That should do it