They don't seem to really care to be honest with you.
In the Dark Apostle series they basically break into a Necron tomb world to rob it more for the power of the technology held within, rather than want or worry of the necrons themselves.
I'm sure Chaos don't see the necrons any more of a threat than anything else however. Chop them, shoot them, grind them into dust beneath the banner of the true gods applies just the same to robot, alien or space-cow.
"Ambition is the biggest killer of Momentum"
Starting afresh with my Chaos Space Marines.
Project Log Link (Placeholder) -
Project Rebuild, Reboot, Recycle:
Stripping, rebuilding and remaking my Guard army.