First, set up your base quickly and get the ODST research from your Barracks ASAP, as well as the MAC Cannon research from the Field Armory. Note that there are two Forerunner Supply Elevators (basically Supply Pads) to the left and right of your base. Send one troop of marines to each to increase your resources even more quickly.
When you've upgraded your MAC Cannon and the ODST research, unlock the nearby airlock and move to the one on the opposite side. You have 30 minutes to release all airlocks, so keep your ODST units coming. When you reach the shielded Covenant Bases, simply unleash the MAC Cannon on it and finish them off with your troops, after which you can re-establish your own base (with a few Turrets and Supply Pads) on that base site.
The 20th Black Box can be found directly north of the Portal, and is even visible when you start the mission. You'll walk near it during the mission, so don't forget to pick it up. As for the Skull, you'll need to destroy the three Scarabs and look to the north section of The Flood area.
Remember that once you've opened up all air locks, opening the ones opposite to each other, the mission will be successful, and you've beaten the campaign!
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