The par score of 25,000 can be obtained on Normal if you make it under the par time, which is 20 minutes. You get a 3x bonus on points for par time. It's a short mission, especially with a vehicle, and can easily be run in about 15 minutes.
You'll want some of the easier skulls on to get the multiplier high enough. Also set competitive score on, so you can see your progress with score and time.
Getting kills without dying builds up your killstreak and earns you more points. Multikills also give you slightly more points.
Famine - Weapons dropped by AI have half ammo. It's a vehicle-heavy level so this isn't an issue.
Fog - No motion tracker.
Tough Luck - enemies always go berserk, always dodge, never flee. Running over enemies will be harder, that's about all that will really affect you.
Iron - Dying restarts the level. If you die, save and quit so that the level won't restart and you can resume with points intact. Doing this will reset your killstreak, however, meaning you'll get less points per kill until you build it back up again!
At the start, hop in the driver seat of the warthog with a turret. Let a couple marines occupy the passenger and gunner seats. Any kills they get while you are in the vehicle will count toward your score. Drive through the level letting the marines kill everything and running over what you can. Be wary of sticky grenades.
Once you get to the barrier, let the gunner take out the choppers. If they only kill the brute and don't explode the chopper, you'll have to do that yourself. You get extra points for destroying enemy vehicles, even if they were never occupied. Once everything is dead, drive up to the barrier to trigger the drones. Back up and let the marine gunner take them out. Don't let them get close or they'll board you and start hitting you. Destroy the barrier and any brutes hiding behind it.
At the bridge, ditch the warthog at the gap. At the area with plenty of marines fighting a pack of enemies, try to get the kills before the marines do. There's more BR ammo inside the building on top of the hill. A phantom will come in and drop off more enemies. You can use the turret in the building to take out the phantom turret and the grunt on a turret. Unlike in Halo 2, phantom turrets don't give you points, but you might as well eliminate the threat. Rip off the turret and take out the brutes. Watch out for the chieftain with a fuel rod cannon.
After they're dead, a wraith will appear. Get close so the wraith doesn't attack and shoot the brute on the turret. Then, board the wraith and hit it enough to kill the driver but not destroy the tank. Commandeer it. Take out everything until you get to the two unoccupied choppers. Leave the tank and get into a chopper. Destroy your wraith for points. Destroy the other chopper. Don't destroy the warthog, as human vehicles will take away points.
From the top of the jump, you can snipe the wraith on the bridge with your chopper. Be careful in this next area, as packs of choppers can be dangerous. Watch out for the shade turrets. There will be another wraith that appears on top of the bridge. Make sure you destroy all vehicles, and not just their occupants.
You can get your chopper past the blockade on the bridge to help take out all the brutes at the second barrier, buts it's a little tight with all the blockades so you might have an easier time on foot. Destroy the barrier to end the level.
I finished with 35,000 points, so there's definitely room for error.
Edit: The achievement didn't pop for me, so I ran through the level again using the same method (got 40,000 this time, less marine kill-steals I guess), watched the full ending cutscene and it popped at the postgame carnage. Achievements for this game have been very buggy. Just to be safe, watch the full ending cutscene.
Unfortunately, the par score achievements in this game be can stubborn about unlocking. Personally, I had to play through the final level several times for the achievement to unlock. You may want to wait for a patch if you don't feel like testing your luck.