The flood ALONE could take on both universes and win. They are the ultimate killing, infectious, and just disgusting parasite. Nothing would stand a chance against them in terms of strength. So combine the flood with the UNSC, Covenant, Forerunners, Prometheans, all fighting together? The 500+ Spartans could probably fight one faction of Star Wars. I'd say the clone troopers. The only thing that could stand a chance against the flood is likely the droids. The Jedi are powerful, VERY POWERFUL. But I don't think even they can stop the forerunners. Let alone the flood.