I’ve tried those fancy shaft clams and vice grips with zero success. If you are replacing the strut, take a grinder and form two indents opposite each other. Grind each side a little at a time, checking the size using the jaws of a box-end wrench. Cut the grooves just wide enough to get the jaws to fit, the tighter the better. Once the box-end wrench fits, you should have plenty of torque to get that stubborn nut off.