They make large spanner wrenches that have an adjsutable arm. Not sure if the local parts store carrys them. Again, be sure that any force you are applying to loosen the nut is being applied in the UP direction. That bad boy is torqued to 150 Lbs-ft.
Since it appears the nut is now dinged up pretty bad I would suggest as in a previous post a pipe wrench.
Also, before you put it back together, invest in a set of reuseable teflon IC seals. I think I got mine for Texas T-Bird and that is the best $25 I ever spent on my SC.
Ed