Get some penetrant on it.. If you have Kroil then use that over anyother penetrant.
Depending on the bolt diameter sometimes after penetrant has been put to it you can use a prick punch and a ball peen tap it loose. If that doesnt work you can hit it with some more penetrant and let it soak some.
If you are still stuck then grind it flush hit the center with a punch and then use a small drill bit and keep stepping up the bits..
If you are worried about touching the threads in the hole because your pilot hole is a little off center then use a pencil grinder or a dremil with a bur bit.. Work the bit around and cope out the entire body of the bolt.. Eventually you will work it out..
Stainless gauls easily so always put anti seize on stainless.