At what extent will a 2,500,000 volt Stunt Gun harm an attacker ??

Most "stun" guns are worthless. You have to be within arms reach of your ailant. For most people that is too close. Stun guns scare people well but I have taken them and put them on my chest and leg and yes they cause a muscle spasm and a little pain but I was not "stunned". A taser on the other hand has a 15 foot shot for the civilian version. You shoot the attacker drop the taser and run and hope he can't get up for a while, which they usually don't. If a firearm is a viable option for you I would look into that.