If by the word "entire" that also includes the Flood at the height of its power, then the Flood alone would be an apocalypse to Star Wars, sort of like how what the Vong did to the Galaxy, only worse. There comes a point in Flood evolution where it is mathematically impossible to hold them at bay--as holding them at bay requires the use of resources, resources that the Flood in turn consume and turn against you, adding to their army while taking from yours--and the denser the victim civilization's population is, the faster the flood will grow. In this scenario, at the height of power, the Flood has already consumed the entire Forerunner civilization and all of its technology (this is to say nothing about their Precursor masters, that's another apocalypse entirely), as well as many other galaxies (source: Silentium). The Forerunners themselves would be a colossal threat to the Galaxy, just take everything they had and put it into the hands of the Flood. At that point, the Flood have already surpassed the point of no survival, where the mathematical impossibility of holding them back has been reached. And here we're sending them to feast on one of the most biologically/technologically dense and diverse universes in fiction? It's a numbers game at this point. As for their Precursor masters, all they have to do is subvert or disable all technology in 2/3 of the Galaxy like they did to the Forerunners and really all Halo needs is the Flood to win. Star Wars' A-game is found in far too many superweapons, nearly all of which can be subverted by the Precursors' tech-hacking abilities. Star Wars relies on one-offs, without those superweapons its useless against the heavy hitters, it has too many of its eggs in too few baskets, where the heavy hitters of Halo are all super weapons in of themselves.