Unless you posted in the wrong place, your rear shocks should be held on my bolts going through the rubber sleeve sideways. The shock will rest in teh bracket, then the bolt goes through w/ a nut on the back. Shouldn't have to hold anything on the shock. You might have to hold the nut or bolt head, but not the shock. Other than holding the nut to keep if from spinnint, they should unbolt, pull out, and install the new one.
If you have Xreas suspension stop and take it to a shop.
It sounds like he is asking about the top nut, not the bottom shock mount.
You just need to get a good grip on the shock body while you are twisting it, or have someone hold it while you loosen the nut.
If you have shop/mechanic gloves or anything that will provide better grip, they will help a lot. There are also those wrenches that tighten using a rubber strap so you don't damage the shock body.
Top of stud accepts a hex wrench as others have said, but it is a PITA to get your hands up in there to do it right. I had much better luck with a pair of vise grips on the shaft itself. I used a piece of thick rubber to keep from damaging the shaft (like a black rubber bungee cord maybe).