To remove the top nut:

With car still fully assembled, put car on ground, raise front of car slightly so you can maneuver a large vice grip (one with nice sharp teeth, curved jaw) under the dust boot and clamp onto piston shaft, carefully use impact wrench from above, use short bursts, just break the torque on the nut, don’t back the nut off much or you will lengthen the height of the strut assembly making it harder to get out from under the fender (while still attached to lower ball joint). Also this is a good time to take a pipe wrench to the cartridge retainer ring/nut, again raise car as needed to allow access through the coils. After you have spring compressor in place(and spring is compressed), spin the top nut all the way off, and Bob's your uncle.

To install the top nut:

During strut assembly, make sure the tapered part at the top of the new piston shaft is perfectly clean and free of any oil or grease, same for the matching inside tapered area inside the upper strut mount, assemble turning the top nut down, threading it until it just begins to make contact between the threads and the locking part of the nut, if possible leave spring compressor installed still compressing the assemble by about an inch or so, then swing assembly under and back into shock tower, remove the compressor. Now finish all other assembly and put car on ground with the full weight of the car on the tires (maybe even somebody kneeling on the front bumper, someone sitting inside too?). Take top nut off (don’t worry nothing will come apart as long as weight of vehicle is on the tires) and oil its threads with very light oil, do same for piston shaft threads, don’t bathe the parts in oil though, you don't want a pool of oil in the top mount, but you do want to apply oil all the way down the threads, use a small brush if needed. Put nut back on piston shaft, getting it just started a turn or two on the shaft, no more, take impact wrench and using full continuous non stop power, drive the nut down and it should tighten up very nicely. If it does still seem to be slipping (which I have never had happen when following the aforesaid instructions), quickly stop and continue using only short blasts.

I have done this many many times, with never a problem.

Also you can take the new top nut and using the discarded piston shaft, maybe in a vise, use impact wrench to drive nut on and off a few times to loosen the locking mechanism up a little, this will reduce its bite on the new piston shaft, causing it to have less turning force on the shaft. This can help those who have already assembled and are having problems, provided you can get the nut off, but do use some light oil. I have never had any nuts come loose after having oil or this method applied.

Edward --
Edward's ~ '92 Gold Brick