Respawn Time: 16 min approximately for place holder. I had to clear the spawn about 7 - 8 times to get The Avatar of Pain to spawn. I did it easily with 75 lvl monk, with no merc and my good looks. Hope this helps!
If you run up the stairs on the southeastern tower, you'll run into this guy. His PH is a Construct of Pain. There are several other guys up there with him so some clever use of camera angles and bind sight is helpful to pull (unless of course you have a FD puller).