I still wouldn't say that this achievement is very easy. It's still very frustrating. Well, it's Halo 2.
Just like the previous levels with the Flood and Brutes on it, for Par Score you want to play it on Heroic with as little skulls as possible. I mean, too many skulls and you are screwed.
I did it and recommend only 3 scoring skulls on: Fog, Jacked and That's Just Wrong. Non-scoring ones, I only used Birthday Party and Piñata.
The weapons I used were the sword and Carbine (and the Beam Rifle at the very end to kill the last Jackal sniper but you can use the Carbine as well). Sometimes it's hard to get headshots with the Carbine due to the Flood's head location changing from soldier to soldier. If after hitting the usual head spot they don't die, try to quickly shoot around the body, rapidly circling it if you can't figure where it is. I tried to use the Carbine whenever I could for 2 reasons: 1- saving a bit of the Energy Sword's energy (not necessary after you know where to find the spares throughout the level); 2- that way they could be revived by the little Flood forms and I could kill them again for more points. It's risky but you can try that if the situation in a room is under control.
I tried to kill everyone and double checked the rooms for more enemies. One of the most frustrating parts is at the last elevator (not the grav lift). In that room you will have Brutes, Grunts and Jackals, one of them being a sniper, ahead of you and eventually Flood will come down behind you. My strategy was to delay the elevator checkpoint to make it less frustrating in case I died. You do that by constantly hitting your sword or punching the air as you go up. When the elevator is about to stop, a little before that, you stop hitting the air and you will gain another checkpoint at the top.
As soon as I got into that room I killed the 2 or 3 Grunts (it varies) and stayed behind cover. Then I took out the sniper and other Jackals from afar if possible, dodging the Brutes fire. I also threw all my grenades at them, because I had Piñata on, so when fighting the Flood with the Sword they will most likely drop a lot more so you can replenish. Be warned that if you get doubles or triple kills while throwing your grenades, more Flood will come in the first waves behind you. Before charging the Brutes up close I waited until the 3rd wave of Flood came down (they are not infinite; none on this level is). Then I killed the two Brutes and moved on to the next room where I exchanged my Sword for a new one (it's in the middle of the room).
On the last room, be careful not to be overcome by the enemies. I killed the Jackal sniper from across the room, then moved to kill the 2 Brutes on the upper right side then did whatever else I could, taking 1 enemy at the time. Sometimes you won't be able to reach the second group of brutes (where there's usually 3 of them) before the Flood kills them.
The Sword is your best friend on this level, but you gotta know how to use it. Usually, there's also a few spares you can find scattered throughout the level, which is nice. If you find yourself with 2 or more Flood charging at you at once, try to escape, jumping on things and to a different floor level than them. If that doesn't work and you decide to face them head on, I recommend jumping backwards in the air while keeping the red aim before/after each attack. That way you can charge and flight each time (hit, jump backwards, hit, jump backwards...). If you miss though, they will hit you, so it's risky.
This is another of those missions which require patience and persistence. I got over 7k when I finally pulled it off and that was converted to 15k because of the times two (2x) multiplier for playing on Heroic. The game didn't register it though (thanks TMCC!) Then I did it again with the same skulls and got over 7k again. With the bonus multiplier my final score was 14k.
If this solution helps you in any way, please leave a comment and a thumbs up to let me know. Thanks.