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 mythic. With these settings I did a very different approach, while I will explain at the last paragraph. I managed to finish the level with 10,531 base points. I beat the level in 13:11 which yielded that glorious 3X multiplayer. I ended up with 31,593.
So my suggestion is to kill what you need to obtain the points to get you to 25,000 with the 3X multiplier. From the start I walked backwards and along the beach and took the path up to hunters. I killed everything, of course, and deactivated the security door. Now from here I went to the newly opened door across the island, but instead of going down normally to the bottom, I took the overshield fall. You can watch my video on Upload entitled "Silent Faller". Basically in the room to the right just after the security door is a spot you can fall from, land safely at the bottom, then fall again directly onto the overshield right next to the cartographer. Then I ran up normally to the top and ended the level. I will provide a video of this fall. 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.