This is, in my opinion, the easiest Par Score achievement to get, solely for the fact that there are so many enemies it is almost impossible not to get it on Legendary with no skulls. Quick tips on what I found important:
1) Grab the Shotgun ASAP. You want to keep it on your person the entire level. Don't worry about running out of ammo, there are enough Shotgun Flood.
2) Keep the Assault Rifle until you come across the Rocket Launcher towards the end of the level right before you grab the Key. There's a part where the Flood storm you from around a corner where the Rocket Launcher is a necessity.
3) Frags are you friend. But always be wary when throwing one close range, since there is a huge tendency to create a chain reaction, especially when it comes to "Pregnant" Flood.
4) Try to speed run this level as close as you can. I ignored a lot of the flood towards the middle of the level and just booked it to the elevators. Just chuck grenades while backwards running to gain extra points.
5) Don't die. Or rather use the save-quit trick to not have a death on your record. It helps your score a lot.
6) Kill the "Pregnant" Flood first, always. They are very helpful for taking out the other Flood forms before you even get close. This is a life-saver in some parts of the corridors. Then focus on the Shotgun Flood, then any Weapon Flood, then No-Weapon Flood, then the swarm stragglers, unless your shields are down, then focus all your energy on the swarm ones, as they can kill you in a second.
I personally completed this level in 30 minutes and had well over 75k in points. This was on Legendary, no skulls, and no deaths. In the beginning I tried to kill everything in my sight before the second elevator and already broke the 25k cap, so I just rushed the rest. So there's a ton of room to get the score limit before the end of the level, so good luck! Any thoughts and comments are much appreciated!