Well I think ultimately it's your call as MC whether you want to allow it to work on groups, but personally I wouldn't unless there were some particularly appropriate circumstances. (It would definitely make an interesting 'advancement' version of the move though, like you say.) There are already moves for dealing with crowds and there is a reason the Hocus and Skinner get them and the Brainer does not. That said, I don't see anything in the move's description that suggests it needs to be close range -- it just says "if they can see you," and specifies that you don't have to interact. For me the only sticking point would be whether or not the Brainer has to know somebody is looking at her in order to use the move -- since they presumably do not know the location of all of these snipers.
If the idea is that the Skinner holds it together long enough that the Brainer is, fictionally-speaking, Going Aggro on the individuals one by one, then I would suggest having them roll for each of them -- after all, it only takes one miss or a few 7-9s before that situation starts getting a lot more interesting. If the Snipers are aware of each other and one of them starts screaming and holding his head while blood spurts out of his nose, the others might decide that shooting sooner than later is a good idea. Then again, a perfect series of 10+ and you have one truly epic brain-fucker on your hands.
Also in terms of 'treating them as a gang' -- gang size is mostly only used for dealing Harm or taking Harm. In the case of that particular move the harm is ap, so the extra armour wouldn't really make a difference to the results. Did you penalize the roll or something instead?