[Psientist]

Here's an alternate strategy then.
Beef up a bane lord with the sandals of the crane again, but instead give him a wraith sword or similar (for added HP). Perhaps a holy weapon (if you are up against a lot of undead who won't be affected by the plague), or the default bane sword (if you are up against regenerators). The point is not to give him power to damage, but just constantly heal and bounce around the battlefield.

THEN have another mage cast plague, maybe twice. That way, the purpose of the bane lord is to just make sure the battle lasts long enough for the plague to do its job, and it doesn't matter if the necromancer lives or not.