Yeah, don't do the zip lock bag, it would probably suffocate her. I personally prefer a cervical dislocation or anything that severs the central nervous system. I've gassed mice in mass, but to me, it is more of a convenience when you have 30 mice. It looks more painful to them than to just break their necks, which is my preferred way. Just don't hesitate and set yourself to do it and get it done. It will be a fraction of a second and that is it. No gasping for air and no hypothermia, just twitching from the loss of signal to the muscles from the brain.