[i]Just[/i] the Forerunners could beat almost anything that Star Wars has, at worst, the Forerunners are merely equal in approximate strength. Either way, a war with the Forerunner Empire is hell for any Star Wars faction that decides to take them on. ...the Precursors could solo Star Wars all on their own in the biggest curb-stomp battle in history.

The Precursors have star roads, which are near-indestructible strings of light-matter and can form webs multiple AU's in diameter (they'd slip into space near a death star and slice right through it). The Forerunners also have Halos, which can do more than just anti-sentient life destruction, as well as shield worlds and the massive dreadnaughts, etc that would easily go head to head with star destroyers. Covenant would also be a decent threat. Humans... well they can just sit by the side. Flood would work out some way of infiltrating the Empire, etc.