Nightsisters without Acolyte have never been a problem for my Phoenix team. I focus down in order Daka, Talzin, Ventress, Talia, Initiate, Spirit, and finally Acolyte, choosing to either kill or cleanse the zombie (via Ezra or Chopper) when necessary depending on cooldowns and the zombie's health. If there is an Acolyte, I auto-basic until she unstealths. Whether that ever happens is up to RNG and how high the skill is leveled, so sometimes I spend the last 3 minutes of the fight on auto until I draw, but usually I can attack at some point and finish the fight.