The Vikkies can be a challenge to handle as a Rezzer. The mark of Shezuul ignores most defensive abilities, making it harder to outlast them. However, positioning is very important for them. If you can lure one of the Sisters away from the other, they lose a bit of their punch. Ideally, you could lure one into gun range, and take a focus shot at her. When you do start killing one though, commit to it until she is dead. Once Blood dies, the fight should get a bit easier.