The goal for Sierra 117 is 25,000 points. It's somewhat of a tough level for par score, simply because there are less enemies on this level than any others and the par time is low. Still, it's possible to get par score on normal. You will have to use some easy skulls and you will have to get par time or very close to par time.
Getting kills without dying builds up your killstreak and earns you more points. Multikills also give you slightly more points.
Catch - Enemies throw more grenades. This makes grunts more dangerous, so your priority in most engagements will be to take these guys out first.
Famine - Weapons dropped by AI have half ammo. You'll be carrying a plasma pistol most of the time, so you'll have to switch it out often.
Fog - No motion tracker. For most of the level, enemies will be ahead of you. There's a few open areas where you will need to be vigilant.
Iron - Dying restarts the level. You just won't have time to die if you want to reach par time.
Tilt - Enemy resistances and weaknesses are increased. Basically what this means is that you'll want to use plasma weapons for brute armor, and projectile weapons for flesh. Trying to BR a brute's armor down isn't going to work.
Tough Luck - Enemies always go berserk, always dodge, never flee. Enraged brutes are easily out-meleed and are less threatening. It's still easy enough to hit brutes with a charged plasma shot. I noticed grunts still fled, but if it means they flee less, that's a good thing. Easier headshots.
BRIEF WALKTHROUGH AND TIPS:
You'll need to be familiar with the level as it is on normal, because you need to make par time. Know where weapons are and know where you're going.
Starting off, you've only got a pistol and an AR. In the first group of enemies, take out the grunts with pistol headshots and swap the AR for a plasma pistol. Take out the grunt on the phantom's turret. Use the plasma pistol charged shot + pistol headshot combo on any brutes. If you run low on ammo, spikers are good for taking out grunts, but they won't do much to a brute unless you take their armor out with a plasma pistol.
The first BR is going to be across the natural bridge that you find a brute holding a soldier on. It'll be next to dead marines to the left. Swap your pistol for the BR. If you can get a hold of a carbine before this point without wasting time, do it.
After the "Cortana moment" occurs and you exit the cave, there will be dead marines to your left with BR ammo.
Progress through the level getting headshots on grunts and jackals and plasma pistoling the brutes. Be quick! Swap your plasma pistol for another frequently.
You'll come to an area where you see Johnson and some marines running across a bridge. In this area will be a phantom next to the cliff edge. If you're quick enough, you can combo the Chieftain riding it and get some decent points. Otherwise, kill enemies and wait until it flies off. To the right of where the phantom was is a slope with a downed pelican and some more BR ammo if you need it.
In the final, large open area, there will be many enemies. Stay on the move from cover to cover. There's going to be a Brute Chieftain with a gravity hammer on the dam, he'll either be on the upper level or the lower one. The building with Johnson has some carbines if you need a precision weapon. After saving him, phantoms will approach and drop off enemies. Take care of them with headshots and grenades as soon as they drop. One phantom drops enemies off across from the building Johnson was in. Sometimes these enemies move up to the middle of the dam or sometimes they stay put. You'll have to kill all enemies to trigger the pelican that picks you up and ends the level.
This method does not have a lot of room for error. I was able to get about 27000 with 14:30, although that could be improved.
For the par score achievements ALWAYS WATCH THE 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.