could be a coincident, but after 14hrs of trying to get this mob to spawn, I found I had more free time between cycles as I had fewer named mobs on list, when I would get one. I decided to not just kill the PH but the mobs in both rooms connected to hallway where he spawns also. The very next cycle he spawned.
the other tower servant as well (directly in front of you as you go thru the west door).
The very next time he spawned.
I also had the same thing happen with the Soulmage. After 14hrs of killing the PH, I had more time and decided to kill the tower servant that is directly in front of you as you go thru the west door. The very next cycle he spawned (at foot of stairs).
It appears there are some contingent PH's.