Got no slot for the screw driver? Maybe try a dremel with a cut off wheel to make a slot. Can also use those oscillating saws with a metal cutting blade to cut a slot but have to be really careful. Duct tape or duct tape over some folded aluminum foil or cardboard around the head can help prevent hitting the inner rat fur or elephant hide liner. Won't stop a solid slip that hits but can prevent a minor bump from doing any damage.
Break Away, or other penetrating oils can loosen those if given some time. Using a torch or a heat gun or even a strong blow dryer to very carefully heat the bolt, then spray with break away will cause the penetrating oil to be drawn into the threads as it "follows" the receding heat. Also the expansion of heating followed by the contraction of being quench by the penetrating oil can help break the nut free.
Large enough wrench to provide decent leverage. One person on inside using the widest blade screwdriver you can find, with a big handle. One person outside with a long handles wrench or ratchet handle. There is a certain amount of force required to break it free, any less won't do anything but strip and slip.
You would have to buy a replacement bolt probably from Scamp
but drill through the nut with a drill just a bit larger than the bolt is including the threads. Drilling out the bolt including the threads will get the nut off. Might need to use a punch to gently tap the bolt through. And of course it will be ruined as will the nut.
Good luck with it, post what you end up doing or trying. That is how most of us learned what we know. Either by having to muddle through one of these "adventures" or by reading what others have done that worked, or didn't.
When replacing use a touch of grease on the threads and on the plate where the nut will press.
The torch and penetrating oil can really work well however if you can heat the bolt really hot with a torch, and then spray oil on it there will be a "steam cloud" of penetrating oil created when it hits the hot nut. You don't want the torch near that. I was under a school bus, really large rusty nut, on the frame so I could really give it the heat, and spray it freely. Should have put the torch behind me before spraying. Mustache didn't get that burned and grew back in a couple of weeks. That is all I'm going to say about that