Edit: I have rerun the level and edited my guide accordingly.
I was able to score 45k points on a no-death run on Legendary (no skulls.) I took 11:40.
It should take you about 2 minutes to get to where you melee the elite in the elevator shaft.
In the first room with enemies (after the elevator climb) I assassinated the Elite on the raised middle path, then grabbed a Magnum from a rack while all the Grunts were running around. Use this to kill all the Grunts.
Grab a plasma pistol from one of the dead Grunts to noob combo (overcharge plasma pistol, switch to magnum, headshot) the Elites that come from the boarding crafts. There is one Elite and a couple Grunts from each of them.
You should be 'Noob Combo-ing' every Elite. I would advise grabbing a new plasma pistol from every group of enemies you fight.
After riding the the elevator, you will fight 3 more groups of enemies (one Elite plus several Grunts each.) After these groups will be another ammo rack. Refill your pistol.
The next room will have an Elite and several Grunts. Killing a few of these will cause a couple of jackals to enter. Watch out for the needlers. Shoot Jackals in the arm so they fall back, then headshot them.
After you've killed these enemies, go back to the rack in the previous room and refill your pistol.
If you're frugal with your pistol, you should be able to clear out the entire big room (the one I had trouble with the first run). There is a Jackal on each side of the glass room, along with many Grunts around and in the room. There are two Elites near the end of the halls, with a third showing up at the final door.
Once outside, grab the floating Battle Rifle (there is more ammo to your right.)
Kill the first Grunts as they jump out of their ship, and the few Jackals that run over. Watch out for the Jackal with the Beam Rifle. The Battle Rifle can also kill the Elites if you can't get close enough to combo them.
Don't hesitate to go back for more BR ammo if you need it, or take a Carbine from the Jackals.
After activating the missile, run over to it to free it as soon as you can. This way you won't have to fight the enemies that are coming in.
Run inside and make it through the ship to get to the final cutscene. Feel free to kill enemies if you need them (though you shouldn't). Note: the timer still counts down during the ending cutscene.
Edlt: Could negative voters please comment. This isn't a particularly hard par score.