Have you tried the mole grips on it before, as normal grips cannot grip it as cannot get them on enough ( not enough teeth on the bolt) so mole grips won't lock the whole way round the bolt anyway.
I got one off using a 12mm socket hammered on, but then that rounded off the 12mm bit now that is stuck on one of the bolts and won't come off even though it slides about

will get heat on that though. So only thing I can think of doing is buying 6 12mm socket bits, or drilling them out.