I had a similar thing happen with a rear sub frame bolt on a subaru that froze up. I ended up breaking off a high speed tool extraction bit in it. I tried drilling it out to with no luck, but what finally got it out was some carbide bits for my Dremel like these
I'd suggest boring the middle out slowly with one of the bits listed above.
Once the hole is bored tap in an extractor like thies
If at first it doesn't come out, wiggle the extractor free and bore it out some more. At some point you'll have just some thin metal of the stud and the threads left.
Tap the threads with the proper tap.