Put a pry bar, screwdriver or needle nose pliers in there between the bolt and the arm and try to loosen it when there is pressure pushing the bolt out.
I see you tried the prybar but did you do it with power tools on the bolt? Maybe spray it really good with WD40 and let it soak all night.
I see you tried the prybar but did you do it with power tools on the bolt? Maybe spray it really good with WD40 and let it soak all night.