THIS IS THE IMPORTANT PART
SavePhase1(Addphase3): Here's where RNG comes into play. You have 2 Fanatics and 1 Adherent spawn. Leave the fanatics alive to hopefully transform, and kill the adherent. Oh, and finish getting the shield to 3-4%.

SavePhase2(Addphase4): You get 2 Adherents and 1 Fanatic. IF both fanatics from the previous wave changed into 2 seperate cultists ("Zombie" and "Skele" were our terms) then kill the 1 Fanatic. Let the Adherents do what they do and just keep basic aggro to prevent healers from eating the shadowbolts. At the end of this wave, kill off any duplicate/normal adds that you can.
Don't Collect "Normal", or unchanged Cultists at this point. Let them transform. You will get 1 of each unchanged add next phase guaranteed. Focus on getting 3 changed adds, and as soon as the next wave hits BAM! You have 5.

SavePhase3(Addphase5):This should be your last wave of adds and you should be ready to push her into Phase 2. Ideally, you will have a Zombie, a Reanimated Fanatic and a Reanimated Adherent alive. (We were under the impression that Empowered Adherents did not count towards the 5 types needed so instead of risking it we did not leave any of them alive). This allows you to kill one of the 2 Fanatics that spawn during this phase, leaving 1 Adherent, and 1 Fanatic, for a grand total of 5 types of Cultists.

Phase 2: Immediately after Savephase3 begins, finish off the shield and kill that add. The tank not tanking the adds should be ready to pick her up. From here its a game of keeping all of the adds CC'd or OT'd. We did the following:
Zombie - Kited
Reanimated Fanatic - Shackled
Reanimated Adherent - Left alone/Pally Fear/Taunt tanked to reduce healer damage
Normal Fanatic - Pally Sap (Repentance)
Normal Adherent - Left alone/Hunter Traped

If you make it this far you have the achievement in the bag. Might take a few attempts but you can do it.