In case somebody is wondering, the way to do this is on Legendary without any skulls on. Kill every enemy you see and that should be more than enough (I went over 20k and the par is just 18k) allowing you to even die a few times. Dying nets you at least minus 100 points though. Manually reloading the last checkpoint nets you minus 200 points and for me (I did it once) it also caused the score to stop acknowledging new points for new kills. That was fine though, as of at that point I already had over 25k.
By the way, the wepons of choice are:
- For the inside area at the beginning of the mission: plasma pistol + magnum (noob combo).
- Against the first group of enemies outside and the first Wraith: magnum + sniper/rocket launcher + plasma grenades. Kill the grunt on the left(stick him with a plasma grenade), rush straight ahead to start killing the enemies that are attacking the marines on the other side from the door you came out. Watch for the Elite. He will inevitably get into a Ghost. I usually get close to him and then stick him on the vehicle with another plasma grenade. Clear the rest of the guys if any are still alive. Frag grenades are the best against Jackals (and the plasma pistol, of course). Kill the grunt in the turret on the top left when facing the Wraith. See if there's any other enemies behind the rocks to the right of the Wraith so you can start moving on closer to it with the Rocket Launcher. You don't want any disturbance while trying to hit the Wraith two or three times. Careful with its cannon shots, dodge. Once this part is over, you're good to move on and find the Scorpion. Before that though I usually go back and pick up the pistol or the sniper, whichever I dropped to carry the Rocket Launcher. The Launcher is great, but you will only need it at specific times on this mission. It's better to have a close and precise range weapon (magnum) and another for picking enemies at a distance (sniper). You will find more jackals before you reach the Tank. There are also enemies rushing the Marines at the Tank location. I usually throw grenades to delay them and then rush inside the vehicle.
Take good care of your Scorpion Tank once you get to it. It is your best friend throughout good part of this mission. Make sure it doesn't get hit often.