I have a few ideas... first of all try "rost off crafty" from wurth, this is a realy good freeing agent to get out rusted in bolts etc, then a stud remover. Sykes picavant make a good one, it rescued one of my wifes calipers once.

Then you can use a very thin diamond tipped cutting disk on a dremmel to cut a slot into the offending bolt etc and try a screwdriver.

The last and very very dificult is to drill it out so carefully you are then able to peal the thread out of the hole leaving the other thread intact. I have not been able to do this my self and only ever met 1 person who has done that succesfully.

hope this is a good start!