It would be nice but I get the idea the game does not encourage that from the gm's comment on you have no idea when the next one will show. I think we we wait to long and not make good on killing some of these guys then we fail.
We could just patrol the area let our presence be known and keep a fortified camp site. But I think from other expeditions that the GM is looking for a group to get into the den and lay a beating on them.
I figure they are camped just on the inside edge of the mouth 20 thieves ball park. We get in behind them if any luck we have vegetation on our side there. Go look at scouts advice in the adventure with Krakyn in the medieval extension threads.
Right now I am thinking of a way we can force some of them out to deal with them. We can not take the whole lot of them at once but if we can put some hurt on them then maybe we can flush a few out here and there. I was thinking of a catapult to launch a few nice sized rocks at them. Maybe give them a nice black eye. But I do not want them coming down in force and using it against us.