I'm currently replaying the game while laying as few ghosts to rest as humanly possible. Upon entering spooky hollow I didn't have a plan on how I was going to approach it, so I let myself get a team recommended and just rolled with it.
We all know the dude worshipping the skull is under an astral field. The ward itself is nowhere to be seen, there is no way to disable it and so there is definitely no way to enter the mill. Or so I thought. I was recommended Hypnos, and that gave me an idea. What if I were to sleepwalk inside the mill by using the homeless person as a fetter, then putting the dude worshipping the skull to sleep, bring him outside and see what happens? So I did. After a few attempts of binding and unbinding (since you can't decide on the direction of the sleepwalk) I managed to funnel him into the mill. However, turns out that putting the worshipping dude to sleep is not possible - it has no effect on him. But then I noticed it's possible to drive him insane by using delusion and psychotic rage (that one doesn't come by default on either sandman, you'll have to purchase it). It works. After a couple of goes you will have him driven insane, the level will end immediately and - most importantly - you will not have any new ghosts recruited - nobody, not even Dragoon! You don't need any other ghosts to make this work since the plasma required to cast the abilities is provided by Dragoon himself, when he chases off the humans for you.