loosen and remove the nut while the cars standing on its wheels (cars weight keeps spring compressed and unloads the shock shaft & nut from the top cap)
by sprayin wd40 at nut, leave for abit to soak, grip the nut with a swan neck O-spanner while holding the shock shaft still with an adjustable spanner or tight fitting C-spanner, gently and progressively loosen the nut without rounding off the flat sided shock shaft end.
when you jack the car up the strut and coilspring will progressively extend fully until they seperate from the top cap (cos the end nut is removed). then unbolt the top cap from the chassis.
i prefer to retap/clean the shocks thread end to make assembly & future removal easier.
upon assembly, refit spring & bumpstop, compress spring, fit topcap, fit nut, remove compressor, fit onto car, lower car down onto wheels, recheck nut tightness