Go into Bunnings and get the biggest pair of these you can find:
Then using them, grab the gland nut(the nut with the two holes in it holding the shock in) real tight and undo it. Hold the strut in a vice if possible.
If that fails, use a hammer and chisel to hit a grove in the gland nut, then position the chisel on an angle in the grove and hit it as to force the gland nut to turn and undo. You'll need a vice for this one.
If that doesn't work go back to Bunnings and buy a universal disc removal tool for an angle grinder. They are kind of a 'Y' shape tool with two little prongs on the end, and you can set the distance between the prongs. You may have to grind/file a bit off the inside of the 'Y' part so it clears the shock shaft when you're undoing the gland nut. Position the prongs in the two holes and see if you can undo it that way. I've done this a many times and it has worked until I bent the prongs.
Looks like this:
If that doesn't work grind the edges of the gland nut flat so you can get a BIG shifter on there. The new shocks should come with new gland nuts, and hopefully they will be the hex type so you can use a shifter to do them back up.
You can also buy a gland nut wrench that looks like this:
I have no idea where you would get one though, other than the internet.