When just pushing a breaker bar or wrench on something that's too tight you can strip it. When they are that tight it is best to use something like a brass hammer and hammer at the end of a wrench (put the box end on the bolt) in the direction of loosening it with a series of knocks to break it free. You can use a breaker bar and socket but it's best to use something thats solid like a box wrench.
If the head does get stripped you can use a chisel on the head like this. This will almost always work. Just be sure to hit the end of the chisel with a series of easy blows as possible to help keep from cutting off pieces of the head. If you have a chisel that's a little bit dull that would be better. If this fails you may have to drill it out.
Tip... - 
