Did this on Heroic, with Iron, Famine, and Catch skulls on. Most of it will be in a Banshee, so thrown grenades won't affect you too much. Remember to use save scumming to deal with Iron. Empty vehicles do not give you points. You may also want to wait until someone gets into a turret before destroying it for the same reason. Most of the open areas have spare Banshees, so get one if yours is too torn up. Finally, don't take too long. Time will be your enemy on this.
In the beginning, drop the Grunts with your pistol. I then picked up the shotgun lying there, but if you don't like it then ignore it. Straight across from is a weapons crate with two Beam Rifles. Since they are in the crate, they aren't affected by Famine. Use it to pick off the remaining Covenant ground forces. You should be able to get all of them with both Rifles. Another nearby crate has the Brute Shot, so if you want you can grab that. One of the Grunts had a Fuel Rod Gun, definitely take it. Once you have whatever weapon combo you prefer, get in a Banshee. One balcony has a Jackal sniper, another has Brutes with shots and Grunts with Fuel Rod Guns. Waste them all. Then land on that rooftop and collect ammo for your FRG. Destroy the ground forces and Shades, and as said above, letting units get in the Shades before destroying them is what will get you points.
Once the tunnel opens, two Banshees come out. Waste them both and head into the tunnel destroying the sniper tower in the process. In the back section, there are ground forces and an unoccupied Wraith. If you wait a bit, a Brute should get in the Wraith. This is more dangerous, but also allows more points. Your call.
Back outside, a Phantom is deploying an Engineer Recharge station. If you destroy the "squid house" it will detonate the engineers the Phantom still has, which might let you destroy it. In any case, quickly destroy all enemy units. Remember that you need to open this door.
Another tunnel, a few ground enemies, and a Phantom deploying a squid house. This Phantom you should always be able to kill. Back outside, one AA Wraith to your right, one Phantom deploying a squid house in front of you, another Phantom behind it dropping a second AA Wraith. The Phantoms escaped from me, but you should destroy the AA Wraiths before your team can. A balcony once again has Grunts with FRGs, so be careful about them but collect the ammo once they're dead.
Once again, you need to open the door, and this time you need to go into the tunnel on foot to clear it out. This is probably the hardest part. There are Brutes, Jackals, Stalkers, a Chieftain, and an Engineer. The Engineer will of course give them more shields, and the Chieftain has Invincibility as an Armor Ability. Either kill the Engineer or lure enemies away from it. If you quickly run up and fire your FRG into the initial mass, you should get several of them and detonate the plasma crates. Otherwise, just be careful and kill them all. The building at the entrance of the tunnel has a health pack, the override is in the building near the end.
Once done, back in your Banshee. Through the Tunnel a Scarab is waiting for you, along with Phantoms and squid houses. Fire at the squid house in front, and you should be able to kill that Phantom. Destroying the others should clear out other ground forces, and might damage the Scarab. There are also two Banshees flying around to attack you. Be very careful about switching Banshees out here, the Scarab might fire at you. Even if it doesn't kill you, it might send you new Banshee flying away before you can get it. Kill the Scarab as fast as you can, as you're probably over time by now. Then get to the door and open it. Fly in and blow up the closest squid house to trigger the cutscene.
I was overtime by the time I finished, but I was able to get enough points. I will admit it was close however, so you really can't afford to waste time.