I kept casting Sense the Dead. There were times when the kitty had popped,
and it was stuck in the wall or wandered off without me seeing it, so I got into
the habit of checking this way. An added perk is that if there are enough folks
hunting in the zone killing the other pesky undead, you might pick up one of the
kitty pops from the other spawn point as well (near the center of the zone).
I spotted one on Sense the Dead radar, and asked a ranger to bring it to me,
and he did!
The placeholders do periodically pop in the tree, and sometimes are very hard
to see. Keep a timer and look around until you find a bat, grol baku soldier and
rarely, a grimling orefinder to kill every 5-6 minutes. You can't just sit there
and wait for the PH to smack you. It won't every time.
Three guild clerics did this quest recently, and I would say that our camp drops
averaged a little over 1 skull per hour. Good luck!