Put your vise grips on it and apply medium turning pressure. Tap the end of the bolt with a heavy hammer. Reverse turning pressure. Tap some more. Repeat back and forth. Don't hit too hard or try to turn with too much torque. I have spent some time ( 30min to an hour) with bolts like this and managed to free them. The key is always patience. Applying oil and pushing it back with your blowgun helps too.