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