*Stares at monitor*XMark said:The Hypnotoad.
ALL GLORY TO THE HYPNOTOAD!
So a C'than? lord dude/whatever can eat a star... what if you throw a black hole at him? Does he eat singularities for breakfast too? I mean... you would probably have to throw it pretty fast... maybe slingshot it off a larger black hole. These guys are flying through space and shooting laser beams and shit. The ability to move a black hole isn't even science fiction in OUR reality.
If you kill a "scrub" necron it comes back... after a while it will stand back up and fight you again. I'm working purely on the assumption that there is some kind of "lag time" involved in the thing(s) getting back up. Would it not be possible to "harvest" the remains of said necron army while its in a "regenerating" state and chuck the remnants into the nearest neutron star? There is going to be a limit to "self repair" at that point. This is also working on the assumption that they could lose a battle... I'm not saying a whole war... and granted they could make more... this is just a way to potentially make less.
On a side note... the humans have Paul Atreides... and no one has ever taken an accurate census of the Fremen... I believe they exist in vast numbers.
TL;DR Hypnotoad! Black holes>C'than guys? Chuck scrub necron "corpses" into neutron star? Paul Atreides and the Fremen.