“nut on enough to protect the end of the screw then hit with a heavy hammer or dead blow”
Not a fan but if this is it I’d consider a heavy dolly (maybe some dive weights taped together?) of some sort topside to give some mass to dead blow against. Would rather crack the fastener free some other...
Any room to back off the nut a bit and put a second nut on to lock in place, then something with a cheater bar to crack the bolt free? Alternatively make a lock nut with two thinner nuts?