How do i flirt with a girl that i find hard to flirt with?

The same way you flirt with the other girls. You just have to build the courage to walk up to her. If you're infatuated with the girl, MAKE A MOVE! If you don't start flirting or show you're interested how do you expect it to get anywhere? And also, if you start flirting with her and she's interested too, you should probably tone down with the flirting for other girls to show that you really like her the most. If you don't, she'll just think you're a natural flirt and she's just another girl you flirt with.