As soon as the Y-Wings appear target each in turn and tell them to wait (Shift-W). Destroy the mines and the T/Fs yourself, watch out for a couple of T/Bs that appear to ruin your day as well! Next lower the the FRG's shields, this will take a lot of time, but be patient, you can do it. Once you have brought the shields down tell the Y-Wings to proceed with their mission (Shift-G) and they will disable the FRG.