We have to think since all these mutants turned villains, they likely make the first move. The way I see it. They break off into serveral forces. The main assault groups would consist of some of their best fighters who attack various government installations across the world. Possible leaders of some of these platoons are Magneto, Cyclops and Wolverine. I am sure there are many other good candidates as well. Another small group would also stay behind at the Xavior Mansion to protect Xavior. To help thin the herd Xavior will use Cerebro to target every humans everywhere and kill them all instantly or at least wipe their minds. Since the mutants are making the first move to take over the world and using this kind of strategy, the number of enemy forces left to challenge them will be pretty small. Then there is the matter of them trying to stop both the assault forces and Xavior themselves. Which would force them to split some of their forces up. Reducing their numbers even more. There is a good chance each of them could end up dead before even reaching Xavior or stopping the assault forces. Even the mighty Hulks can be vulnerable to mind attacks. Especially if they are boosted by Cerebro. Unfortunately though if all the non-mutant villains are helping as well, that means Galactus would be showing up as well to defend the Earth. He might post a bit of a problem. Though probably by the point he or any other aliens arrive to help, there won't be many humans left on Earth to save. If there are any left at all.