If it is fast necrosis then I think you are just screwed. If it is slow, then maybe you can save it. I had a birds nest do that and just fragged the hell out of it then dipped the pieces for good measure. Make sure to frag off pieces at least 1" away from the rot if you want to try to stop the spread. Also maybe take a few pieces from the far side and move them to try to save it.
edit: ps the core of my coral died, but enough frags survived that I was fine.