Only sure way is to get a hotter girl than her and to make sure she sees it.
But girls often go to bars specifically to reject dudes and get attention fix. Call her out on it with bored "oh, another one of those attention whores" look, leave her table and have fun nearby totally ignoring her. I guarantee that she'll elevate you above other dudes, she'll remember you next time she sees you there and she'll even approach you. They hate being labeled as members of "boring gray zone of girls" so they'll do anything to become special in your eyes.
But girls often go to bars specifically to reject dudes and get attention fix. Call her out on it with bored "oh, another one of those attention whores" look, leave her table and have fun nearby totally ignoring her. I guarantee that she'll elevate you above other dudes, she'll remember you next time she sees you there and she'll even approach you. They hate being labeled as members of "boring gray zone of girls" so they'll do anything to become special in your eyes.