How to take down this pesky pretender?

[Loren]

I was having quite a bit of trouble taking down a pesky pretender. He was a wyrm with some pretty good stats. Repeated attempts to kill him always failed. I had a lot of gems, I decided on a different tactic. I wished my prophet (my pretender was immobile) up to some pretty insane levels, gave him 400 ancient ones and an ethereal crossbow with orders to attack rearmost (he had a precision over 40, he should have hit!) He never fires. I script shadow blasts--a few turns into the battle he drops one into my troops, never fires one at the pretender. I tried a few other spells, same effect. I was about to try sending him in alone (the wyrm never had more than a few units and my guy was tougher) when I got a dominion kill.

Other than dominion, how in the world should I take down a pest like this? He was powerful, regenerating, good MR, poison immune and a good spellcaster with a full load of gems. I simply couldn't do anything to him, in time my forces would rout.