It seems that these "Score" achievements are quite difficult but ill tell you how I did this level. Every kill gets you points and dying or losing Marines loses points. These points are multiplied by what difficulty you play on and the skulls you use. To play with score showing, choose to show score in the HUD when you choose your mission. The par for this map is 25,000.
I had my score showing, heroic difficulty, and all main skulls on EXCEPT for Foreign. Keeping this skull off allowed me to use plasma grenades which really helped clear out groups. With these setting, I ran through killing EVERY enemy I came across. I managed to finish the level with just under 9,000 base points. I beat the level in 11:55 which yielded an extra 3X multiplayer.
My suggestion is to melee everything. You can shoot some grunts, but honestly, they wont go down easy. If you continuously melee a grunt or elite over and over, your shield won't drop, allowing you to stay alive while killing. And don't die...using the Iron skull is crucial to your multiplier and dying makes you restart the mission. One last thing: If you do die, press start and restart manually. Letting the game restart not only starts you with negative points, your time doesn't reset.