How to get a corroded bolt off my car?

I have a corroded screw on my back end of my Oldsmobile Cutlass Supreme. It's on the top half of my shock where it uses two screws to secure it to my trunk area. I don't know how long it's been stuck in there but I have newer shocks and that one side is knocking around because it isn't screwed in on both sides. I don't know how to go about getting that screw out. There isn't a head to the bolt, it's flat on the top and bottom like it's already been messed with. How would I get this bolt outta there to put in a new one?