Which nut? Nut 54 is what you need to remove partially to remove the lower control arm from the kingpin. That nut should come easily. It will probably be impossible to get the trunnion bolt #142 out if its froze to the carrier (un-numbered part in #158). The bolt 142 is about .75 in in diameter if I remember correctly. It will be hard to break it. It will be hard to cut it and it is even tricky cutting with a torch and saving the control arm and the kingpin. Since you seem to need 158 and probably a control arm. I would remove the carrier with the control arm as I said. You won't have to remove bolt 142 in that case.
Bosch Master, ASE Master, L1
33 years MB technician