I couldn't find anything about it either. Hmm... Time to think outside the box :)
Oh, I guess I didn't actually ask this, but I assume we don't have clear line of sight down the hallway of the dungeon entrance?
I think what I'm going to do is use my Complex Geometric Estimation skill (mastery 19 I think) to attempt to bounce a shot down the entrance corridor. Not knowing the size of whatever might possibly get hit, I can't roll the damage myself, and I can't know if I succeeded so the skill roll is done by you as well. Should I go ahead and make an attack roll?