Im sure you've heard of the concept of Calibration. A shy girl does not need to be neged.
I still dont know a solid method through this, but this is what ive been doing in its stead.
Game her in a group. Stand next to her and when someone else is talking lean over and say little comments to her. Start building comfort with her.
Another thing you could do is get a wing man to be a total asshole to her by putting her on the spot and purposly pushing her 'shy' buttons, and you rush in there and tell him to back off and bam auto-comfort.
Just remember to take it slow! Shes shy.