Face and limb shots would work, IF you can manage them. With a ballistic helmet and throat guard, that is a pretty small target. Limb shots will almost certainly slow down the attack, but a slower mass murder isn't that much of a victory (at least not for those in the room).
I think this might become more of a tactics question than targeting. A side or rear attack from close range is probably the best bet. If the shooter doesn't know they are being attacked until you have an almost contact shot available, your odds of stopping them are better.
In a dark environment, with a panicked crowd, and gas, the odds of a successful immediate stop aren't great for people more than a few feet away.