looking for tactical advice

[Taqwus]

Well, if he's down to one province, you can probably annihilate them all by wiping out his dominion. Sit on top of the province, preach, surround with temples. Works with Sphinxes and anything else that's not willing to sortie out and break the siege.

You can also use remote commander-killing spells to shut 'em down.

That said, priests help of course, along with mundane commanders bearing Herald Lances + Boots of Quickness, along with death mages -- Dust to Dust, Wither Bones do unresistable area damage to undead. Very nasty. More extreme, there's Solar Brilliance, at the cost of potentially blinding large parts of your army.