It's already been explained multiple times in this thread to a greater degree than I could. But I'll still explain. Simply put, unless you take out the Precursors and the Flood, Star Wars has no chance of winning. And I'm being very generous in them having a fairly good chance against the Forerunners,UNSC, and the Covenant. The Precursors are the equivalent of Gods that actually interact with the universe. Something that can't really be said for Star War's celestials. They could cause massive wormholes, resurrect themselves if they chose too, and travel through different dimensions. Star Wars has absolutely no defense against this. And if the Flood is at their height I.e Keyminds and multiple gravemind with trillions of linked flood soldiers under their control, plus the ability to corrupt non-biological life? All it would take is one planet to be overrun. And let's not even get into Star Roads. And let's assume the The two factions are fighting from different galaxies. The Halo rings could be fired in the direction of the Star Wars Galaxy and then massive forerunner and covenant armada can just move on in and decimate solar systems and glass planets. Star Wars is powerful but in this case It's is just outgunned and outmanned. Infantry wise they would clearly have an edge, but it would honestly never get to that point.