In each mission (two player) there is a hidden objective, they are:

Mission 1: Complete mission in four minutes or less.
Mission 2: Destroy all Trade Federation Carriers.
Mission 3: Destroy all station doors.
Mission 4: Destroy all Trade Federation Sub docks.
Mission 5: Protect all buildings.
Mission 6: Destroy all containers.
Mission 7: Destroy all Sabaoth crafts.
Mission 8: Destroy all defense barracks.
Mission 9: Prevent all Drop Ships from landing.
Misson 10: Destroy "Reavers".
Mission 11: Destroy all Missile Frigate groups.
Mission 12: Destroy Cruiser without using the orbital cannon.
Mission 13: Lose no Clones.
Mission 14: Remove all of Toth's shields.
Mission 15: Destroy all turrets on Destroyers and Flagships.