You can get this on NORMAL following my method closely! The par score is 25,000 and the par time is 15 minutes.
Use these skulls:
Anger - Enemies fire faster and more often.
Catch - Enemies throw more grenades. Flood can't throw grenades.
Famine - Weapons dropped by AI have half ammo.
Fog - No motion tracker.
Iron - Dying restarts the level. Try not to die. If you do and still think you can finish the mission with par score, then just save and quit as soon as you die. Then resume the level with points intact. Doing this will also reset your killstreak, meaning you'll get less points per kill until you build it back up again!
Jacked - Can't board ground vehicles. There are none on this level.
Streaking - Shields decay at a slow pace but killing an enemy recharges it some. This is a great skull on this level because flood are easy to kill and you'll have an overshield most of the time.
WHAT TO DO:
Skip everything you can until you fall through a tunnel and enter an open room with a piston where you see your phantom shooting at flood. Use your camo to run past enemies. Kill enemies if they're in your way, but don't stop to clear a room. There are a couple points where you need to do something to progress:
-Activate the moving platform. You may not have to kill the enforcer, but I did so anyway. Just pick up a plasma rifle, get underneath it and fire away. Sticky grenades speed up the process.
-After the moving platform when there's another enforcer. Killing the enforcer trigger the large doors to open. Sentinels will pour out. Camo to get past them.
-Anytime there's a piston. Shoot it or melee.
Try to pick up a BR and/or carbine by the time you get to the open room with the phantom. Once you do, get critical hits on the flood in this room (shoot them where the tentacles are in their chest, it should take one shot) It's important that you try to keep their bodies intact.
A short amount of time after you enter this room, some infection forms will enter the way you came in. DO NOT let them hit you and pop! It's okay if a few do, but the more there are the better. Let them reanimate the dead flood. You may need to lead them toward the bodies.
Now comes the fun part! Run around the room, killing the flood again and avoiding the infection forms. They'll reanimate the bodies over and over again, giving you points each time you kill them. You should be going for multikills, which earns you more points. More infection forms will drop into the room periodically. There are some carbines by the edge in the back if you need ammo.
Eventually the infection forms will stop coming and you'll have killed the flood for the last time. You might get a checkpoint, too.
I entered the room with about 1.5k. Afterward, I had a little over 21k points, and I was at 12:30. I finished the level with 27k at 16 minutes.
Continue with the level. Kill mostly everything until you reach the elites holding out. In order to trigger the ending, you have to kill ALL the flood that spawn. I once had a single flood that hid behind a rock and prevented the level from ending, so be sure to look around if everything seems quiet but the level doesn't end. You should acquire over 25,000 by the time they're all dead.
WATCH THE END CUTSCENE
Unfortunately, the par score achievements in this game be can stubborn about unlocking. I've had to play through some levels twice. You may want to wait for a patch if you don't feel like repeating levels.