Dome of Light/Walkthrough
From Halopedia, the Halo wiki
Quickly kill the incoming Covenant troops, which are fortunately easily repelled from the Base. Start setting up your base with the regular buildings; You should know by now what you need to get the basics out there. Definitely give your Turrets the Missile Launcher upgrade as you'll be dealing with a lot of aerial Covenant ships.
When your resources allow you to, request a Rhino Tank at the Landing Pad directly right to your base and move it towards the first Rhino destination. Little support is automatically give, so you'll want to back it up with a Scorpion or two.
Also start building a small army of Scorpions to deal with the Covenant Base right at the second marked spot. They're moderately strong, so you'll really need around three Scorpions along with the Spartans to deal with this base.
The third marked position looks like it can not be accessed, and it actually can't. You'll need to press UP on the D-pad, order a transport and then move several troops (preferably including a Rhino Tank already) to the platform with the 3rd marked position.
You can find the 6th Black Box to the west of marked position 3.
If you want to find the Skull, destroy 50 Banshees and look behind your base around the hangar - it will be available there.
Make sure you have a few hero's and marines at all three Rhino's, and you're good to go. You'll get a message from Anders that she needs two more Rhino's in positions #4 and #5.
Before anything, quickly defend your base from the incoming Covenant hostiles and secure the area around your base and spot #1, #2 and #3. It helps to build a few Wolverines and Scorpions to assist the Rhinos on spots #4 and #5. When all Rhinos are in position, the force field will finally give in, allowing you to shoot the Mac Cannon from the skies. You have 40 Rounds which is plenty to destroy.