Damn bro u stripped that nut up real bad. I hope you get the idea and know not to let it get that bad next time.
Make sure you use the right size socket always and use some WD-40 or something similar if the bolt is tight or kind of seized. Also, once you see it starting to get stripped like that, do not continue. Use a vice grip like suggested before and crack it, once it's cracked use the socket and you're home free.
Right now with the nut already cut and abused like it is in the pics, you're probably gonna have to cut it off like also suggested.
P.S. Using the screwdriver to break the nut is a good idea, but I suggest also trying a chizzle, the bevelled edge and thicker shaft might make it easier to break.
Good luck!
-Shankar