You are right, the quick burst of torque is what gets the nut out. You can try PB blaster (a penetrating catalyst) on the nut the day before to loosen the nut and see it gets the job done. It is the stem that spins that you need to hold and I'm not sure if you have clearance to get a pipe wrench but you could try a vise grip. If not, you can carry the strut to a shop to have the nut removed. They might do it for free since it won't take but a few seconds to dislodge the nut. Good luck!