Does the bolt move at all? The best way is an impact gun. If the impact gun won't back it out, Keep driving it back forth while spraying PB blaster into the threads you can get too. It will come eventually. Obviously get a new bolt, and see if you can fix the threads in the crank with a tap. If not, you're going to be relegated to a helicoil or an insert. Try the helicoil first.
Kurt