This was considered a regular mission, but if a solo Hunter-Demon came here, they wouldn't be able to finish in a day or two. There were several ghouls, and a lone Hunter-Demon would have to find a way to lure them out; otherwise, it meant digging graves – a laborious task. If luck wasn't on their side, they might inadvertently uncover all the ghouls at once.
If that happened, with their strength severely depleted, the mission would most likely go sideways. To play it safe, they could wait for an easy kill, but that would inevitably waste a great deal of time.
With multiple people working together, it was different. They could simply kick down doors and dig out those hidden ghouls, dealing with all of the problems in one day.