I have a set of those made by Irwin. They should work. The shock nut at the top is a normal thread and turning left will loosen it.
Take the Bolt-Out that is the right size and gently tap it in place with a hammer. Make sure it is really biting onto some of the stripped nut before proceeding. Then use an impact wrench or ratchet to loosen it. The metal shaft of the shock will try to spin. Put a vice grip on that to keep it from spinning.
Option B would be to just grind it off seeing as you are replacing the shock and need a new nut for the shock.
If it were mine I would grind it off using my air grinder and a conical bit like one of the ones pictured here.