A Countess of Pain has two separate locations where it can spawn:
( -170, 25, -667 ) /waypoint -170, 25, -667
( -358, 3, -802 ) /waypoint -358, 3, -802.
They are both spawned the same way. Clear all the faeries in the general area and a group of faeries will spawn at the location listed. Clear this second group of faeries and there is a chance that a Countess of Pain will spawn in a third group.