Nice to know I'm not alone in this. Only solution I've found is, once I've gotten the head to go on the statue, use fag mode and run into the room, place the head and leave. Disable fag mode and complete the level, some of the bishops will eventually follow you out but at least you won't have to kill them all.
Another interesting thing on this level is, entering the Heresiarch's room, if you can kill one of the slaughtaurs with a fire weapon right in the doorway, his charred remains will block the doors open and you can run in and out at will. Run in far enough to trigger the next opponents to appear, then run back out and shoot at them as they walk past the open door ...like a shooting gallery. They can't come out since, apparently, no sprite can walk past the remains of a dead sprite, so they will just parade back and forth, occasionally taking a shot at you.