Best option - find a (LARGE!) socket that will fit and unscrew it.
'I'm desparate!' option - position a flat blade screwdriver near to one corner of the nut and 'knock' it round. This ALWAYS works (as there's no real limit to the amount of force that can be applied), BUT it may damage the nut to the point where it can't be re-used.
phil_saunders@bigfoot.com
Phil Saunders
September 2008