Update: the par score for Metropolis is now only 9,000 points. You can play on Normal with a handful of skulls activated or Heroic with a couple skulls and you should have no issues beating the par score.
I did this on heroic with 4 skulls: catch, famine, fog, that's just... wrong. I finished the mission with 26906 points and over par time by 47 seconds (2.9x multiplier).
I also died 3 times:
* Watch out for that camo elite in the first on-foot section!
* Don't get shot in the back by your rocket launcher teammate if he survives the tank ride - I recommend killing him.
* Don't get run over by your teammate in the warthog while sniping/rocketing the wraith if he survives.
My first run at this had the iron skull on (5 total). I died while boarding the Scarab, save & quit, resume, and finished with 38,000 points, but I didn't get the achievement. Be warned, these scoring achievements are still buggy. Many have commented they did not earn the achievement after beating the score.
Grab the Scorpion Tank and let your marines get on board. As long as they're on your tank, you'll get the points for their kills.
Head over the bridge, destroying anything in your path.
Avoid running over your marines that jump down from the left railing of the bridge (oops).
Once you've taken out the tank, beware of phantoms (covenant dropships) flying parallel to you. Try to shoot off their turrets (lots of points!) or their cargo.
Continue on, destroying ghosts (7?), banshees (2).
2 wraiths will come out of the tunnel you're headed through at the same time as 2 more banshees start attacking you.
Once you need to exit your tank, pick up a plasma pistol from one of the first jackals you kill and start comboing the two elites headed for you. Watch out for the jackal sniper on the far wall!
Grab a beam rifle from the ammo box before going into the circular room.
At this point, I back-smacked the jackal sniper, shot the grunt on the turret, and hopped in the gauss warthog. My marine driver did a couple loops while I killed things (including the 2 ghosts which come a little later). My driver was having some issues, so I got out and headed to the next section on foot.
Snipe the two jackal snipers then pick up the rocket launcher laying on the ground in front of a small piece of cover.
Destroy the wraith with the rockets (it will take all 3) while not getting killed by the two patrolling ghosts.
Snipe or highjack the ghost(s) and then boost through to the next section.
You'll meet up with a warthog shooting two ghosts. I destroyed the two ghosts with my ghost, then boosted up and to the left, avoiding the jackal snipers, like in this speed run video (bottom of the guide).
Kill the two snipers, highjack the back wraith, destroy the other wraith with yours, then destroy the phantom's turrets and ghosts as it drops them.
I followed the speed run guide here too: head upstairs, take the rockets from one of the marines and go out to the first platform with rocket/BR ammo. From here, board the Scarab.
To complete the mission, you only need to kill the two pilots (red elites) and the white elite downstairs. Instead of running down immediately like the speedrun, I killed all the enemies on top and then proceeded down to take out the last three elites with the plasma pistol and stickies (I burned all my rocket ammo).