FINALLY completed this last night on my mesmer.
One of the ramps leading down into the courtyard has a pile of wood next to it, and another ramp has a stack of barrels nearby. If you hop up onto one of these, the summoned undead won’t swarm you (you can still be hit with poison though). From there you can just focus ranged attacks on the acolytes until they go down.
I guess it’s also worth mentioning that my character was level 72 at the time, but wearing crappy level 67 gear.
Good luck, and don’t get too frustrated!