You'll need a score of 25,000 on the level Cortana to get this achievement. Hitting par time will net you a 3.00x multiplier. For this method, however, we will forget about time and instead play it safe and slow.
Among players, this is routinely voted the most hated map of Halo 3, so I'll try and ease this pain with a more detailed walkthrough that will guarantee par score on the first attempt.
Keep in mind that getting kills without dying builds up your killstreak and earns you more points. Multikills also give you slightly more points.
For this guide, play on heroic.
SKULLS:
Catch - Enemies throw more grenade. Doesn't affect the flood.
Iron - Dying restarts the level. If you die, save and quit, then select resume. You points will be intact. Doing this will reset your killstreak, however, meaning you'll get less points per kill until you build it back up again!
Famine - Weapons dropped by AI have half ammo. Weapons that are scripted to be in certain places may have full ammo and you'll definitely want to carry those around, especially the energy swords.
Fog - no motion tracker.
Thunderstorm - Enemies will upgrade in rank. All this will do is increase the number of shielded elite flood. With plasma rifles, you'll tear right through them anyway.
Tough Luck - Enemies dodge more. The ones you'll be reserving grenades for don't dodge anyway.
WALKTHROUGH:
In the beginning there will be many flood infection forms along with dead brute bodies. If they infect the bodies and you kill them, you'll get points. If the body is still intact and they get reanimated, you'll get more points. You can use this to your advantage throughout the level.
In the large room where you have to make your way up top, be careful. You'll get swarmed if you rush up. Take care of the flood at the bottom and any that come toward you. Pick off what enemies you can. Try to grab yourself a sword and a plasma rifle. Slowly make your way up. Lure the elites and brutes into your cover so that the pure ranged flood don't snipe you. Once you got most of the infected taken care of, kill the ranged forms with well aimed plasma rifle shots, kill the tank forms with the energy sword, and kill the stalker forms whichever way suits you.
The next area has dead bodies on the ground. They'll get flood-ified. If you want extra points, shoot them in the chest so they go down intact, then let infection forms reanimate them again. There's some flood bubbles around you can pop to get more infection forms.
When you drop down, stay in that little room and take out the brute and elite flood at a distance with your plasma rifle. Most will rush you. Once they're dead, take out the ranged forms. Then move up with a sword and take out the tank forms. Try to get a bubble shield in this room.
There will be carriers next. A needler will take them out and kill the infection forms they spawn.
The next area can be annoying. Take it slow. If you can't find a plasma rifle, a carbine can snipe unshielded flood. Spikers aren't half bad, either. You may want to actually avoid flood getting reanimated at this point, so melee dead bodies until they break apart. The bubble shield you picked up earlier will be reserved for if you get swarmed. Pop it and use your energy sword. Prioritize shielded elites that have weapons. The ranged forms here can be overwhelming. If you've got sticky grenades or incendiaries, use them. Try to get a plasma rifle if you don't have one.
In the next room, there are carbines right at the entrance that you can use to take out the flood on the level above. Some of them may rush you. There should also be a plasma rifle with full battery to the right. Destroy the bodies of elite/brute flood. You should be close enough to par score already that you won't need to farm them for points. As you progress through this room, more pure forms and elite/brute flood will spawn. Find a side tunnel to take cover in. Kill anything that rushes you, then poke out to kill the pure forms.
You'll enter a large open room with tons of ranged forms. Progress along the side killing everything. There will be a few plasma rifles on the ground. Proceed through the doors and watch the cutscene. Or not.
When you exit the hallway, there will be cloaking equipment to the left. Grab one. Kill everything in the room before hitting the button.
Destroy the three reactors. Be somewhere secure before the explosion, because I've had it knock me off the level before. Go through the door and jump down the hole.
Cloak. If you know where to go, then go. If you're like me and forget every single time you do the mission, back into the hallway, turn around, and kill everything coming toward you until Cortana points you the way.
There's not too many enemies in the way as you proceed to the end of the level. At this point you should have more than enough score and can even skip some enemies. Help the arbiter kill everything, go to the pelican, and end the mission.
I had about 35,000 base score, so I probably could have turned off a skull or two and have been fine.
WATCH THE END CUTSCENE. The achievement may not pop if you skip it.
TL;DR version:
Heroic. Catch, Iron, Famine, Fog, Thunderstorm, Tough Luck. Dual Plasma rifles & energy sword. Take it slow. Let infection forms reanimate flood for extra points. After the reactor, make a run for the ending if you have enough points.
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.