when it happened to me i welded a nut on ( cover the rest of the block with something splatter cant burn throught thick cardboard should do) had been blasting it with anti seeze oil for a while before hand too.
once you get it moving undo then do up a tiny bit spray some anti seeze in and carry on like this , job done mate, dont be to heavy handed or your in the shit, take your time.
as for extractors, being a engineer these do work but if they snap 9 and a lot do your fucked if you can get the bit out.
look at it this way, it you could undo it when it was soild how much chance do you stand with a extractor? not to say it wont work, i just would waste my time with them, not on a head bolt