I think he has the type of hold down that is a plastic piece at the base of the battery with a bolt going into the battery tray and the nut spot welded to the tray. The problem is very tight access to get anything down there. Yeah a bunch of PB Blaster and it might be possible to get a small vice grip or something down there to hold the nut in place. Sometimes an air impact wrench will be able to turn the thing out. I think otherwise it might be time to buy a long drill bit to reach the bolt and drill the head off, unless you can get in there with a cut off tool. Once you get it out, you can either buy a new tray at the dealer for $10 (done that), or drill a hole or two to use that long bolt type hold down. You just have to look at it a couple different ways and scratch your head. Can you cut an access hole in the inner fender or something to be able to grab hold of the turning nut? Sometimes if you try a few things and then go have a cup of coffee, you figure it out.
By the way, I always coated those bolts with anti-seize to make sure they would come out again.