Know how to diffuse a grenade
Things become considerably more tricky when it's a one-on-one situation. It's a rare woman who enjoys being left alone in a bar, and it's strict girl code that if one woman is uncomfortable, both will leave. Women can be counted on to adhere to this code with religious tenacity.
Thus, you are left with the grenade, a woman who can detonate with frustration/neglect at any moment, ruining even the most valiant of efforts.
Your course of action? Well, first off, don't treat her like a grenade. If she feels like she's the bad egg you've been ordered to keep warm, she is going to fly the coop. Lame puns and mixed metaphors aside, don't babysit this person! Engage her: dance with her, buy her a drink, ask her questions, and be interested in her because she is a flesh and blood person with feelings and opinions and it won't kill you to be a decent human for a couple hours.
In other words, remember the charm we asked you to tone down before? Now is the time to turn it up!