You can go back to the calm lands, but I believe the Ronso's may sell Holy Water, as long as you have Some Holy Waters, you might be able to stand a chance.
If you happened to have done the side quest in the calm lands, capturing all the fiends in the calm lands, then you should have enough items to equip one person with deathproof, which will help.
Another thing that will help is watch her hp. She has 3 forms, when her second form is nearly dead, you need to have anyone who doesn't have deathproof to be zombied. Then she can't auto-kill that person, which she will.
Also, If I remember correctly you can use holy waters to customize Zombie Wards.
Hope that helps some.