Had this happen recently.  Used a grinder to grind new flats on the bolt head
and then an impact air wrench to brake it loose (it was on there).  Just keep
grinding a little and then comparing it to the socket you want to put on
there (6 point if you have one) and you'll get it.
Hope this helps.

Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242