She knows you know she has a boyfriend
Does she? Does she know that you know she has a boyfriend? If not, make sure she does. You don’t want to hit on a girl who is confused thinking you’re trying to hit on her, because you don’t know she’s seeing someone already. Make it very clear and obvious that you know she has a boyfriend.
Why hide behind the bushes, dude, if it’s a gun that’s pointed at you, it will definitely pass through. So stand up, face it like a man. And flirt. Talk about her guy too. Let her know that he’s no threat to you. Ask her how he is, while you’re flirting outrageously. Accept the fact that she’s going out, that will make things a lot calmer and fun for you. After all, you’re trying to get a girl with a boyfriend, and hey, you are trying to steal her!