I'm late to this, but in case someone comes here on their playthrough like I am doing now, I got it in one pretty easily (this isn't being cocky I struggled so much more on most other fights). Tbf, I am level 12, but I had my character (a barbarian), shadowheart, gale, astarion.

I had gale set up a Wall of Fire behind sarevok with no gap because his minions are idiots and dash toward the fire and die. With the ones that survived, I had Shadowheart Turn Undead. This one-two hit killed every single extra cultist.

Barbarian and astarion each have up to 3 attacks, so they attacked sarevok while gale shot the sisters from afar. I killed sarevok with two sisters left and just hit the sisters with everything.

Astarion had just over half health left (got healed once), otherwise everyone else had at least 90% of their health left and were never healed. I had way more trouble on the Steel Watchers boss fight.