I assume this is the same as on my 98 Jetta. You need a wrench that will reach down into the recess and turn the large nut on the shaft, and at the same time be able to keep the shaft from turning by inserting an allen key wrench into the shaft top. This is why a standard socket wrench won't work, no opening for the allen wrench. And most box end wrenches won't reach far enough down into the recess.
I bought an extremely cheap set of air wrench sockets at Menards. I took the one I needed to a local auto shop, where I do business, along with a piece of steel rod (an old cut-off tire lug wrench) and asked if they could weld the iron to the socket at a slight upward angle, leaving the middle of the socket open. Before doing this I ground off the exterior finish on the socket and ground a step in the iron end so the two pieces would mate somewhat and the bare metal would weld easier.
This gave me a nice home made wrench to turn the nut that has a hole in the middle through which I can insert the allen wrench. It worked fine but still took a lot of effort to bust one of the two nuts loose.
If you have some really good air tools try this: hit the strut top nut with good penetrating oil and let work a couple days. Then just try to spin the nut off with an air wrench. In the old days getting the nut off a standard front shock absorber presented the same problem. Turning the nut would turn the shaft and there was no easy way to hold the shaft. An air tool would typically take the nut right off. The shaft has enough inertia that it will allow the nut to come loose with the very fast impacts from an air tool (why penetrating oil would be a must). I've not tried this as I have no air tools, but it might work.