how do you beat the witch king when you and idrial are fighting him alone? he keeps on destroying me. normally i would have used morgul decay, but i used it on him before when you fight him like 8 times. what should i do?

It is best to have a well-levelled berethor and hadhod/elegost, as well as a very good idrial. Idrial should have learned both Aura and Power of the Valar, and Berethor should be immune to fear and have at least his 3-strike attack (Ecthelion Wrath). If you choose Hadhod, then he should have mountain shield, and if you choose Elegost, then he MUST have True Shot, or he will be worthless. You must also have plenty of curative items and items than hinder enemies.
If you use perfect mode, it is best to use Roots of Wrath, which stuns enemies that are immune to sleep, stun and paralysis, and it also damages enemies greatly. Also, Idrial would greatly benefit from an Ancient Elfstone of Spirit Enhancement, which effectively multiplies the power and effectiveness of her healing skills and Loudwater Fury by x10.

I have above, told how to defeat him both when Idrial alone (Ancient Elfstone of Spirit Enhancement) and how to defeat him later on (in Pelennor Fields).