OK. Here is the battle: Galactus has entered the Halo Universe. The the client races of the Covenant turn to the UNSC to lead them against the World Eater. Galactus will have ALL of his Heralds with him. The Halo energy will not work on him, as when he entered, a tactical pulse (similar to what was used at Installation 00 at the end of Halo 3) was used, and it showed Galactus actually gets stronger by feeding on it. All the Halo factions are united, Covenant, Human and Forerunner. The Flood are assumed destroyed, but if found, even they will side against Galactus. As said above, Galactus has each of his Heralds fighting for him.