Try a hand held impact driver. The kind you smack with a hammer, you hold it like a punch and it turns with the impact of the hammer, and sends an impact jolt into the fastener. You may have to grind a straight slot into the head of the bolt and use a regular screw driver bit instead of the phillips bit. Impact drivers work wonders. Or you could just try an old screwdriver and smack it with a hammer while applying rotating pressure.
SP