Remove the rear wheel and the fender - you will have much better access to the bolt. I think that will get you where you need to be.

A section of metal pipe can be used to extend the length of your wrench to get more leverage.

Is there a nut on the bottom? Can you get a wrench on that?

If you decide to use heat (a torch) then you must remove the rear wheel and fender and clean the grease off the chain, etc., or it will quickly catch on fire. I don't think this is a good DIY method.