This fight is significantly easier if your party has an aerotheurge expert with some basic sneak skills (actually the sneak skills may not even be necessary). You can sneak around and use tornado to remove all the cursed fire, except for some remaining in the office to the left and up the stairs of the main entrance doors.
Then cast nether swap on the revenant in that location, but outside the office so as not to aggro the revenant in the office. Then cast tornado on the remaining necrofire in the office; now you're done! Its safe to now kill the revenants. Since all the fights are 1-on-1 its particularly easy; unfortunately, you get no XP from the fights, but if you can at least use their corpses for source points.