i feel your pain!
i changed my motor mount a few days ago and that 8mm bolt was stuuuuck! try this: hit it with some liquid wrench, then strike it with a pin punch and a hammer. then come back later and hit it again with liquid wrench (lots). finally use a legitimate breaker bar and socket and slowly apply pressure until it budges dont apply sudden pressure as if the bit slips it will ruin the bolt.
try some comet abrasive powder on the bit head to grip. if all else fails try a SMALL torch and dont make the bolt too hot as it will ruin the heat treatment on the bolt. (after you clean up all the liquid wrench).