Really tight Victa 2-stroke crankshaft nuts do need impact to break them loose.
In the absence of a rattle gun, another way that nearly always works, is to use a long 1" AF ring spanner. 'Angled' type is better than 'cranked', but both will work.
With the mower turned on its side, use the toe of your boot to hold the spanner on the nut. Hold the blade disc edge tightly with one hand [gloves help], and hit the spanner hard with a steel hammer. About a 32oz ball peen hammer usually works best.
Failing that, as Joe says, those 1mm cutoff discs are brilliant, used carefully.
