Stalemate; it'd turn into Hive Pantheon/Vex Axis Minds vs the Precursors. Destiny's infantry vastly outmatches Halo's, however. The Cabal alone could probably take both the UNSC and the Covenant; supplemented by Guardians, Vex, and Hive they'd get trashed. The Hive and Vanguard would fight the Forerunners and Flood, beating both because of the Light and Oryx.