Update - A recent update has lowered the score requirements of most levels. I will leave my old guide just as a general guide, but some new notes.
- I would now recommend doing Legendary with no skulls on. This should give you more than enough wiggle room, even if you don't get par time. Refer to the below guide though as its still relevant.
While Shand's guide works, I found this level pretty easy by playing on Legendary with just 2 skulls on (Famine and Fog). I chose Famine because on this level you're drowning in human weapon ammo and plasma grenades anyway, and the Fog because essentially every enemy is right in front of you.
To start I rushed to the door so I'd be ahead of the marine, and jumped over the one at the armory, I reached Keyes in about 50 seconds.
For some reason, I didn't get the pistol in the hallway (dunno if this is specific to difficulty or what) so in the first room I stuck to the Assault Rifle. There should be a grunt and 2 elites in the back corner. If you kill the grunt, marines come from the door distracting the elites, making it easier for you to kill them. From this point on I would stick to Assault Rifle and Plasma Pistol as your weapons.
The only other really specific tips I have are to not hesitate to spam plasma grenades (as the enemies drop quite a few) and to grab the pistol about halfway through the level (its after you duck under a door, swap your Assault Rifle for it.) If you aren't too great with the plasma pistol combo though I'd advise saving the grenades for the elites, though the plasma pistol is definitely easier for the 2 story room.
I was able to finish the mission in 11 minutes with a score of 35000. I died 4 times and plasma grenades were almost half of my kills. I looked at my score and honestly would have had enough points even with no skulls on, but it would be cutting it a little closer (I would have earned 27000 with no skulls, so it would depend how good your streaks were)