It should rattle off with a decent impact gun.
Leave the car on the ground so that there is load on the top mount.
Then when nut off, check to see if it is the strut thread or the nut that is damaged.
If it is only the nut then a new nut will fix the issue, if it is the thread on the strut then you will have to run a die down the thread to fix or stick on a preowned strut.
If you have to use grips on the ram of the strut, make sure that you have it well protected with a thick cloth.
Good luck.
Preowned struts and nuts are in stock if required.
Alex.