There's at least another working purging wand. Without going too much into it, there's a quest involving a dragon near the south east side of the map. Do it and you shall be rewarded.
If all things fail you can kind of teleport your characters over to safely trigger the dialogue with Bishop Alexander without getting rid of the shriekers first. I don't recommend it because you might screw up the quest trigger to actually finish the act. Plus, the fight is hard enough as it is. Cutting off your retreat route isn't the smartest thing to do.
