There might be some copyright infringement in posting the actual stats, but lets just say they are INSANE!
The good news is, that they are also slow as molasses and you can generally stay away from them and work towards phase out. The Nightbringer actually blows up when he dies (large blast template *shudder*), so even in death, he does nasty damage. I suppose the C'Tan ARE technically Gods, so they SHOULD be at least somewhat tough to kill, eh?
If you want more specifics, check out the Necron codex.
That is all.
CrimsonShadow