The Forerunners could already solo them. It's just overkill to add Precursors. Halo, hands down. The Precursors can solo DW and 40k; they'd freaking curbstomp Star Wars. The Flood would be unstoppable once they infected a Jedi or a Sith and gained access to the Force. Hell, they'd probably infect the Force too. The UNSC couldn't do much, neither could the Covenant. At most, they could reinforce Forerunner and Precursor fleets and ground forces. Even if SW [i]somehow[/i] manages to beat the Forerunners, the Precursors, and the Flood, then they just activate all 12 Halos and kill everything; not that they need to.