So, I'm in the middle of a game, and we paused due to a rules argument that got vicious and a little personal.
Here are the relevant stair rules . I'd copy-paste them, but the paste doesn't seem to be working in Chrome atm.
The situation : Silhouette is standing on the top of a staircase, with a ranged weapon. Nanok is standing at the base, adjacent to a Blood Ape that is attacking him.
The Overlord's claim: Line of sight is traced through the base of the staircase, therefore Silhouette cannot attack the Blood Ape because Nanok blocks her LoS unless she has the Precision talent, which she does not in this case,
The Player's claim: The rules for staircases say that she has LoS to all spaces adjacent to the other end of the staircase, and there's no caveat preventing this from working. Thus, by a literal reading of the rules, she has LoS to all 9 squares in the other zone regardless of what's in any of them. This is partially justified by 3D geometry; if she was standing 10 feet above Nanok's square, he couldn't block her vision relevantly.