As soon as you scare off one person, three Ghostbreakers arrive to make your life miserable. They enter on the second floor via the central staircase. A good trick is to use a Surge set to target opponents. Set the trap on the pop machine just across from the door.
Once you get a bit of a scare into the Ghostbreakers they are pretty easy to take care of, but watch your haunters and run a hit-and-run campaign until they are really scared. Otherwise they may well bite back and banish a haunter or two.
Also note that you only have to scare off the Doctors to win the assignment. However, by the time you have scared off the female staff and the Doctors, you've most likely emptied the building anyway.
You should be able to get a Triple Jack-O-Lantern Scare Rating by following the above Phantom Of The Operating Room walkthrough.