we had some trouble with this trash for a while when we were learning spider wing until an inspiration hit me, and no i didnt crit a flash heal on myself...
how to kill faerlina trash, all of it, every time, with zero deaths:
http://www.bjhanish.com/faertrash.jpg
step 1: Form your raid groups 1 and 2 as follows:
1) warrior warrior warrior priest priest
2) warrior warrior warrior priest priest
step 2: send in a hunter to icon the acolytes to your warriors, and have him drop an exploding trap in the (top left) doorway.
step 3: hunter shoots a cultist and runs into the pit room, where dps destroys all of the cultists asap. They run allover the place and dont really need to be tanked anyways.
meanwhile: your warriors in 1 and 2 [dark browns] are on the other side of the little doorway walls from your priests [white], group 1 on the left, group 2 on the right. The warriors tank the acolytes there while the priests cast prayer of healing (healing their warriors through the walls), and once the cultists are dead the warriors bring their acolytes into the pit in quick succession after eachother to be grouned into mince-meat.
also- make sure the priests tuck themselves into the corners tightly so the cultist knockbacks dont hit them, should one happen to wander nearby