Another thing that helps a lot with this battle is the nature mage's Spirit Embrace. It'll help bring up your lightning resistance up to begin with, but it will also help your melee characters out a bit in another way.
The dark wizards have one curse on them called system shock. It's like a souped up Blind curse, and it can seriously cripple fighters (I learned this by doing this battle as a solo fighter without any backup). They only seem to cast it if you're up close to them, so in general your rangers will be safe from the effect of it. But it's only a curse, and Spirit Embrace will stop it from affecting you.
From cradle to coffin... Shall my wickedness... Be your passion