Even the reds and blues themselves would be hard to kill, they did take down an army of Texs. Plus the freelancers and all the Spartans. Cheif with a Sniper, BR, and a couple of grenades could probably execute order 66 all by himself. But when you bring the might and power of the Jedi and Sith from the old republic into the mix, it would be quite the war. Lord Vitiate is basically a god, but then again so are the Precursors. But we know so little about them so it's hard to say.