Finally unlocked this. Having read through the above posts, I wanted to add a couple of things.
- staying ahead of the tank at the beginning is a must. If you let it get too far ahead it can steal several hundred valuable points.
- rush to the first overpass after the beach, take the left side stairs, and gun down the enemies as they spawn on the right. I believe there's about 8. Toss some grenades, aim for the head and grab the multi-kills.
- in the scaffolding section before you get in the tank, left side, make sure you mow down the enemies as they run out and for cover.
- I had 8,250 points by the end of this section.
- once you're in the tank, take out the enemy tanks fast. You should get a multi-kill and at least 500 points per tank. If there's any foot soldiers nearby, hit them for an added bonus.
- in the underground parking area, don't shoot all the enemies. let one of the occupy the parked tank on the right as it's worth more points.
- once you ditch the tank, prior to entering the building/bar, make sure you get the easy multi-kill on the three soldiers inside
- there's another easy multi-kill as you're crossing the bridge. There's about 6 soldiers you can drop. Toss a couple of grenades and head shot the rest.
- after the cut scene, head shot and multi-kill the two guards before the door
when you reach the hangar, spot all the enemies, don't engage immediately. Grab the 40MM launcher and grab as many multi-kills as you can. There's two guys right in the middle, two on the catwalk. Then wait for the remainder to move close together and BOOM!
- note: I had 17,250 points entering the final hangar
- in the final hangar, use the same method. The soldiers are clustered close together. There's two below on the right walking, two in front not moving, and another two at the far end on the balcony. If you time it right, you can kill all six and get over 1,000 points.
-after the first wave, use the 40mm launcher to drop the guys at the back of the room.
- after you've cleared the room, go through the door. If you're missing a few hundred points, you have one last chance. After you take position behind the turret, a truck will drop off four soldiers. If you time it properly and aim well, you can get all four and a minimum of 400 points
The key is: MULTI-KILLS!!
Hope this helps.