The covenant would glass earth, with little effort defeat the Reef. The Fallen could possibly band together into a formidable opponent, but their ships do not have any heavy weaponry that we have seen. So they would be defeated, if with some trouble. Then fight the cabal, probably would need help from the UNSC to run them out of the system completely. The Dreadnaught would probably be obliterated by MAC guns. Glass the moon just to be safe, at least to seal the hive inside it for a while. Now the Vex, they would be unpredictable, because they don't have ships, they would teleport onboard. SPARTANS would deal with them most likely. The Vex might be capable of defeating a few fleets from the inside. But with the eventual glassing of Venus that would push them back. I have no idea how Mercury would play in. Rasputin might catch the Halo Universe off guard but for the most part, he is an unknown variable.