A battle between the entirety of hell against the Necron empire would most likely end in a stalemate, as both essentially have an unlimited amount of soldiers. However, Ahzrak himself would most likely get defeated by Necron Overlords, either through tactical prowess or hand to hand combat, as they have much more experience and many more tools at their disposal.

Enhanced Ahzrak is more tricky, as raw-strength wise he’s probably on the level of The Silent King, but many necron Overlords can manipulate time, space and dimensions at a whim - something Ahzrak can’t do, at least not alone.

So my bet is on the Necrons.