I like to use a big (12") adjustable wrench on the flats at the end of the strut shaft---it is a lot stronger than any 10mm open end, even a Snap-on.

If the strut is an old one, being replaced, Vise Grips , preferably the kind with curved jaws, on the shaft, between the spring coils, may be necessary. Vise Grip is a brand name. Other similar locking pliers are inferior.

Never remove the big nut completely unless the spring is secured in some way (spring compressors) There is considerable energy stored in it, even when the strut assembly is fully extended, as in off the car, carrying no weight.