use some bigass pliers that aren't all torn up, still have sharp teeth. grip that bastard as tight as you can with both hands and break it loose slow. if its rusted on or stuck somehow use some aerokroil and let it soak about 15 minutes. id probably cut off whats left of that stock wire and put my own ring terminal on if its just hanging on like you say.
also, you didnt want to use a 12 point wrench. a 6 point will fit perfectly around the nut assuming you have the right size, wont strip it as long as its not already stripped. make sure you get a new nut once you get that one off //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif
if its really rounded off where even that wont work, the easiest way i've found is to get a tiny chisel and slowly hammer a groove into the nut in the direction you need to take it out. eventually it will cut into the nut, catch, and break it loose. of course you want to be especially careful using a chisel around your alternator and wires...