So, you need to use summons. Before going there you should already be immune to poison clouds and have some skeletons lv2 and other creatures.
Try to find spell from fire or storm which will help to provide an additional Dmg
try to find another type of staff
I don't like it with necromancer but it's not impossible. If you are immune to poisonous clouds certain things cannot harm you.
I played with necro and won it, and I also won the main boss when it was poison immune. You just have to diversify your build.