Whack it with a 2lb hammer and pin punch inside the socket head several times . Hopefully it will get broken a bit loose. Or you could pound your 12pt bit in with one of those manual impact drivers that you hit with a hammer.
Get all the rest of the head bolts out first before destroying your tools though. If these do not break it loose go ahead and try the torx bit it might work. If all else fails drill the bolt head off & pull the head straight up over the bolt stub. You should be able to get the stub out with a stud extractor or even vise grips. Cheers Dan

__________________
It's always something simple
91 300D 603.960 (from japan) 194K
92 Toyota Diesel Landcruiser HDJ81-t 116K
02 Golf TDI new head courtesy of PO
87 300D
97 BMW 525 TDS Wagon 5spd
bunch of Onan and other diesel generators