It seems like like the poison DOT fire is reduced by armor so the highest armor rating you can get while not being vulnerable in root armor will solve it.
However I would wait until you hit the plains to do these. They are full of great loot, but RNG will burn you in some of them; there can be over 5 guys in a pile in some of the caves and having access to grain means not only fire resist but higher tier foods and armor and weapons as well.