Stripped bolt head on front knock sensor

First order of business...I have got to thank Montypython for his perfect re-creation of my experience with the driver side knock sensor replacement ...I laughed so hard I almost forgot about the pain from the slab of meat right hand of mine
Now I have to deal with a stripped bolt head on the front (passenger side) knock sensor. I cannot seem to get any kind of tool in there to loosen up the bolt...any ideas besides dynamite or a 20-lb. sledgehammer? The previous owner obviously tried this one and (expletive deleted ) and after he stripped it left it to rot. So...I've tried to get a vise grip in there to no avail...any other tricks?
Thanks in advance and any help you provide keeps my slab of meat right hand from turning into a bloody stump!