Start off on the Metropolis level, and eliminate the Marines that board the Scorpion, you'll be glad you did. Cross the bridge as you normally would and destroy all the Ghosts and Wraiths that you encounter. When more Marines join the fight, though they are your own guys, take them out as well before reaching the end of the bridge. At the end of the bridge take out the Banshees that attack you but leave one in the air. Also, make sure to hijack a ghost before entering the tunnel.

Lure the Banshee into the tunnel by shooting it periodically to keep it interested. When you reach the first barriers, take out the Warthog and the Marines or they'll destroy the Banshee. Continue to lure the Banshee past the second barrier and eliminate any Covenant resistance.

You'll notice the Banshee will get stuck in the corners repeatedly. The best way to get it out of the corners is to stand behind it and walk backwards until it re-targets you. This will save you a lot of frustration.

The hardest part of the whole process is to lure the Banshee into the small tunnel and through trial and error you should have the Banshee in this position. This is the part where you will either succeed or cry and start over. When you see "Loading done" immediately board the Banshee. If you inadvertently trigger it without boarding then the Banshee will disappear and you will have to start over but if you're able to board it then pat yourself on the back. You've done it!

Navigate it out of the tunnel and fly toward the twin bridges. In the middle of the first bridge you will see an orange cone your Holy Grail. Swap it out or dual-wield it; you now have the most powerful weapon in the game. Special delivery for the Covenant and it ain't no Candy-Gram!