How do you say no to a friend when they want you to go somewhere?

Background: my friend, who I once considered my best friend, wants me to go on a party bus with her for an early New Year's Eve party. She plans to get drunk. I don't want to deal with that. The last time we went out, she got stupid drunk in a restaurant and vomited everywhere. She has a tendency to blow up at me when I say I'm not comfortable going somewhere. I think she doesn't even realize her behavior. And she doesn't care if it makes me uncomfortable. I don't want to babysit and I don't want to drink heavy with people I don't know. How do I say no and leave it at that, and deal with her outrage (which may end our friendship, given how she tends to react)?

Tl;dr--- my friend drinks heavy when we go out. I hate it bc I end up babysitting. How to say no to party?