I played this game for the second time after a while and im playing on hard mode and its so easy my god. Today i faced the nazgul guy in minas w/e with gandalf and i killed him before his turn lmao.
Heres how i did it:

All my characters are above level 65 lmao. I was using benethor, the dwarve guy and eoden and gandalf was there too. First was gandalfs turn i used his lightning strike. Then it was benethors turn. With benethor i had already learned his passive skill of double attack with the cost of 200 sp. Then I used one of his leadership attacks which many people find useless but its so good lol. It's called war cry which lets 3 characters attack by themselves so gandalf, dwarve and eoden all attacked with there most powerful attacks. Then i used war cry again lmao. Afterwards eoden finished him off.

THE END

Heh. I did something similiar. I had Morwen, Berethor, and Eaoden out. I had Morwen do double attack, Rohhiric Ramapage (x2), then had Eaoden use Rampage of the Helm, then used War Cry with Berethor, resulting in the death of the Witch King...well..not death, but you know.

LOL I used Gandalf's Bolt of the Valar, Berethor's Gondor Rampage, and Eoden's Wrath of Edoras (Hadhod was useless, so slow he didn't get a turn) - I didn't get hurt because the only time he attacked, TWK used Fear on Berethor, who is obviously immune