This ones actually kinda interesting. On one hand, the Gravemind is inert currently in the series (since he technically isn't /dead/ until flood have been eliminated as a speices(?)) The Necro's happen to still have one or more Brother Moons active and alive. And from my understanding, Necromorph host bodies more durable than those of the Flood. Ultimately I think that Necromorphs would outlast the Flood in a fight provided that we're talking about the current state of each universe. If we level the field, giving the Flood a Gravemind and the Necromorphs a Brother Moon it could be anybody's game. But if we have a pre-ring detonation flood versus the Necros, I have no doubt that the Necromorphs would be hung out to dry.