I recall Reeve saying they were going to send squads of SOLDIERs to attack Sephiroth, but after the disc 2 closing events ShinRa was to disorganized to get such an attack together.

And if Cloud & co. didn't have the Sister Ray to blow up Sephiroth's shield they could have always blown it up with some sort of heavy materia bomb they went to a lot of trouble to acquire.