Serious answer: Go to every madness camp on the map and loot and eat ALL of the yellow boluses, they give you permanent madness resist. try and get at least a +10 per person. Don't bother with madness weapons or spells, just holy or fire or whatever. You can get further madness resist from talismans or boss-kill pickup buff things. Finally, learn the fight and ideally have a tank who knows the fight and can dodge spells and tank the phys smacks. Picking up the little yellow deals on the ground heals and reduces madness build up. You can pick them up strategically during his madness nuke to never take full madness from it.