Here is how you could get the worlds most powerful weapon, the Scarab gun:
Begin on the Metropolis level, and kill the Marines that board the Scorpion.
Cross the bridge as you normally would and destroy any Ghosts and Wraiths that
you encounter. When more Marines join the fight, take them out 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 repeatedly to keep it
interested. Be careful though, you may destroy it. When you reach the first
barriers, take out the Warthog and your allies or they’ll destroy the Banshee.
Continue to lure the Banshee past the second barrier and eliminate any Covenant
You’ll notice the Banshee will get stuck in the corners constantly. The best way
to get it out of the corners is to stand behind it and walk backwards until it
flies back up.
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 stuck in the
tunnel. This is the part where you will either succeed or fail. When you see
“Loading…done” immediately board the Banshee. If you trigger the loading screen
without boarding it then the Banshee will disappear and you will have to start
over but if you’re able to board it, then good job!
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. By picking it up, you will have
the ultimate weapon that the covenant fear.