i like to put the nut in a vise and use a strap wrench to break it free. then i will compress the spring with my feet with the clutch on the ground and spin it off by hand and slowly release the tension. dont put your fuckin face over it, if you slip and it flies off, it will maim you real bad. lots of tension
if you want to quell your nerves a bit, this tool makes it very safe and easy. i bought one because i have million vespas and i've used the tool a hundred times and is very worth it to me. it will also save you from it exploding unexpectedly. but with a bit of care, you don't need the tool.
even with the tool, i retorque the contra nut with a vise and strap wrench