I'm not eager to give up the breastplate -- Bone Armor does a good job of keeping my Poison Golems alive...
OK, let me just spell out what I'm working with:
Poison Golem with Bone Armor, Vine Shield, Fire Brand, Pendant of Luck, Amulet of Antimagic, Boots of Quickness, and Gore Helm.
The gore helm is the only item I can easily replace, but I don't know of any other helmet that would help here much. The Copper Plate would keep him from getting fried, true, but then he's vulnerable to being eaten by squads of knights or other troops with a good attack.
I guess the only practical answer is to crush the AI before his armies get that large
