I don't have a lot of 'feel' with impact guns so I did it by hand. If you have a small pry bar, you can put it in the slot in the top of the strut shaft. But lay the tip of the prybar across the slot from the side (so the prybar is perpendicular to the shaft, not from the top like you would use a screwdriver). With the prybar sideways, you have leverage. Now you can cinch the top nut up.
Just another way to do it if you dont have an impact gun.
Good luck.