I actually did this room during 2x rare mobs.. and had him spawning in both PH spots.
To be safe method (have Invis undead)
1. Kill the two as you come in the doorway from the main circle (not into his room)
2. Invis undead
3. Then walk straight in (shrink if you can). drop off the ledge right there
4. You will now see the mob roaming (around the lower part of the inner circle (not the two above you)). You will also see the two mobs off the left (one is a PH). Then the others to your right. This method only cares about the ones on the left, even though I cleared for the right also.
5. Stay invis undead until the lower roamer is on top of you and kill it. nothing will agro
6. Check to see if between the PH and the secondary mob on your Left, if that secondary mob can be "pacified or not". If not... be ready to mez it, or off tank it or whatever, but you need it to come to you also, don't let it stand there.
7. wait for the upper roamer to go back up (not be at the bottom of the ramp)
8. Pacify and pull and or pull the PH and the other. it was 50% of the time for me it was pacifiable and other times it wasn't. once you break the right side it doesn't matter
9. Kill the named or main ph, then the add... (give yourself a second so there is space) because named will take longer to kill
10. Invis Undead AGAIN and rinse repeat. keep your self invised to undead whenever not fighting...
I never had the lower roamer be a see-undead invis.. not sure it can be..
Cheers