[ITSv40]

The easiest way to loosen the top nut is with a rattle gun before removing the strut from the car.

The next best way: is to compress the spring to take the pressure off the top mount and then carefully cut with a hacksaw vertically down the nut alongside the strut threads - taking care not to damage the threads. The remains of the nut will then twist off with a spanner on the remaining flats. If the nut is too far down in the top mount dish, you could use an angle grinder with a 1mm cutting blade - but there is a greater risk of damaging the strut threads or cut away the top mount with the angle grinder and then hacksaw the nut. The top mount is scrap anyway, but you need to salvage the strut if still in good order.