Be sure to use a lot of penetration lubrication, you can than try to cut a slit/slot in the screw head and use a flat head to try and screw it out.... chances are that is not going to work.

IMO you may want to go ahead and take a drill and drill bit and drill the head off the bolt, after the head has been drilled off on both bolts you should be able to pull the arm pad bar off the assembly exposing the rest of the threads to the bolt. Hit it wit a lot of penetrating lubrication and let it sit for some time. You can at that point take some vise-grips and thread them out.

as always if your uncomfortable doing this your LBS would love the business!