Good luck, it's a swine to get undone if it's seized. Get some WD40 on it for a while to "try" loosen it off. You may also need the help of a friend. I had to use an oil filter tool to hold the shock itself while the nut was held. Make sure you have a spanner on it that fits....... And I mean fits!! If you round the nut, you might have to get the mole grips on it.
WD-40 is the trademark name of a penetrating oil and water-displacing spray
It does release and penetrate... Its not the best but its a good all rounder.
In 1953, Norm Larsen developed WD-40. He was employed by the Rocket Chemical Company
PLUS GAS seems to work for me.