[thejeff]

MR of 20 isn't enough. Not if he's the only target and you can spam spells at him. You can go with a few mages w/ penetration or more without. Soul Slay, Enslave Mind. Charm or Hellbind heart if you've got those. Make sure you've got enough chaff to give you time for multiple casts.

Thugs of your own with AN weapons. Preferably no-buff flying ones to hit him before he can buff.

Life Drain? Infernal Prison/Claws of Kokytos?

It all depends on what you've got.

Well buffed troops might even work. Weapons of Sharpness, Strength of Giants, Quickening. If you can swarm him, that's 30+ attacks which makes his defense irrelevant. With AP, he's only got effective protection ~12.
Don't worry about protection on the troops, go for high damage.