Nest time use a single hex socket and a decent rattle gun. You will have no problems then. In the meantime you have two options,
1: Heat the nut with an oxy until the nut is red hot, then undo as normal.
or 2: Wind on another nut of the same type until they meet. Weld the two together with a mig and try again.
Also the hacksaw idea is a good one. Just carefully cut at an angle through the nut on opposite sides, this will sufficiently weaken its grip.