[Aleph]

What about Demonbred assassins with annoying items to try to pick off his leadership? I don't know, fire bolas, poison weapons + medallions of vengeance, maybe a black bow of botulf to at least feeblemind his blessers, or perhaps equip your assassins with slave collars and hope his prophet grabs it by accident. While he can take lots of ablative commanders to try to thin out your percentages on assassination, you can probably wipe out those guys with battlefield spells in your hit and run attacks.