So for score, 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! With these settings, I ran through killing EVERY enemy I came across. I managed to finish the level with 12,730 base points. I beat the level in 27:46 which yielded an extra 2.2X multiplayer. Anything under half hour will give you an extra 2X+ I ended up with 29,100.
My suggestion is to melee everything until you get the Warthog. At which point I had a marine on the gun and ran over literally every enemy from then on. With long levels like this, with Iron Skull on, I found it much less frustrating to save and quit if you die. Then resume the level from the main menu to start at your last checkpoint, points intact.