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.