Misaka would win - just let her shield herself with iron sand and then she would just explode it to the outside like shrapnel(similar to massive amount of low-powered railguns), or else she could just make so high voltage near her that if Hei would come in contact with it(does not matterwith what he would come in contact - the resistance that most things offer would be too low to block the voltage) he would just die.
Hei could only win if Misaka was not aware of his presence and/or hostility - he could just assassinate her.