Cross threaded or rusted on is the only way that little 12 lbs of torque nut isn't coming off. If you are having that much trouble, you ARE either going to have to soak it in WD40 (or something similar) or apply some heat. Cutting off the nut should be a final resort.

6 pt is always better to use than 12 pt along with the correct size socket. AND quality sockets that fit properly work best when trying to do mechanic work. Hell, I've even found Craftsman not to fit as well as they should... That really pisses me off too!

And even if you destroyed it, an upper strut mount is not that expensive to replace. It just means that you'll have to drop your strut out to get to it.

Just be sure when you get them all back on not to over torque those little guys.