Well, that ended up being easier than I had originally thought. Thanks for all the advice.
I built a temple next to his fortress which depleted his Dominion.
I also set the tax rate to 200 in his province.
My Pretender God was close by and so I set him to cast elementals and moved him and the retreated armies back (I still had some troops without a commander maintaining the siege). I hadn't researched Lightening so couldn't use this spell.
As it turns out, the AI sent his forces out to break the siege and the elementals and Wardens killed the pretender easily.
Next turn I stormed and there was only one mage defending so it fell easily.
