A simple solution might be to get downward pressure on the strut so there is tension on the nut and then try to loosen it at the same time. You are trying to get it to move up to the next thread. It looks like you could wedge a big flat screwdriver in the gap at the center of the strut tower to push down on the strut. I'm assuming you have the bottom bolt out and this nut is the only thing holding the strut. You also might try pulling on the strut from the bottom to put pressure on that nut.