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 competitive score showing, heroic difficulty, and all main skulls on except for foreign and mythic. With these settings, I ran through killing almost every enemy I came across. I say almost because I did skip the part where you open the door for the Marines in the ship and went through an opened door to progress. I managed to finish the level with 25,159 base points. I beat the level in 28:37 which yielded an extra 2.14X multiplayer. Anything under half hour will give you an extra 2X+ I ended up with 54,054.
As you can tell I doubled the points I needed, but was unsure what skulls I needed. I played on Normal with all skulls on and barely missed the mark, so I suggest heroic with some easy skulls. I highly suggest leaving Mythic off. When you first enter the ship there are 6 sword elites that are MUCH easier to kill with lower health. As usual, melee everything. 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.