This is one of the easier level's to get the par score which is Now 17,000 points. I played on Heroic difficulty, with 6 Skulls on. These were Black Eye, Eye Patch, Famine, Fog, Foreign, and Iron. I was able to get 42,406 Score in 18:14 time. This setup was for 25,000 points which was the Old par score. To make this easier on yourself you can reduce the skulls your using on Heroic. Or keep these skulls and play on Normal. As you can see with my time and score there is A LOT of wiggle room for a now reduced 17,000 point par score. UPDATE At the time I wrote this solution par scores wouldn't add Co-op scores together. Now if you play in Co-op your score and your partners score will be added together, making this easier to do in Co-op.
Here are some tips to keep in mind while going for the par score on this level using this setup:
1. Your Melee attack instantly restores your shields, use this to your advantage, keep close to your enemies so if your shield starts getting low you can throw a bash attack to recharge your shield. You can even bash the tiny Infection Form flood to restore all your shield, so if you see a straggler use your melee.
2. When fighting the Covenant in this level use your human pistol and melee, you want to stay as close to the Covenant enemies as possible. There are no Elites in this level so nothing will bash back at you.
3. As soon as you come across the Shotgun switch it out with your Assault Rifle. The shotgun is the strongest gun to fight with against the Flood. Most flood will die with one shot from the shotgun. There is also plenty of ammo lying around this map for the shotgun so blast away.
4. To maximize your score you will want to find a Balance between kills and advancing through the level in a timely manner. You will want to kill most of the enemies you come across but sometimes if an enemy such as a grunt is running away it might be wiser to forget about the easy points to save 30 seconds. After you hit the par time which is 15 minutes your score will start declining as you are losing your time bonus multiplier. If you complete the Level with at least a 2X time multiplier and have killed most of the enemies you should be well over 25k.
5. Know the level before you attempt this achievement. Hitting the par scores can be tough, making a wrong turn and not knowing the level will ultimately reduce your time multiplier hurting your overall score. It might have been an obvious tip but it should be stated none the less.
6. If you die, quickly hit start then Save and Quit. You can resume the level from your last checkpoint and won't have to restart the entire level. This will save you time and frustration of trying to run the entire level without dying.
7. I would suggest in the options change your Scoring and Time Hud to show Competitive Score. This will show you the level time, your time bonus multiplier, and will show you your Overall score. If your overall score is over 25,000 when you finish the level the Achievement will pop. I prefer this setting over just the score because it gives you instant feedback of how you are doing with not just your score but your time as well.
Please leave any questions, comments, or concerns in the comments and I will get back with you as soon as possible.