You can achieve the par score of 25,000 on The Arbiter on Heroic with a number of skulls active. You will also have to be close to par time, which is 15 minutes. Luckily, almost the entire banshee section can be skipped.
I recommend putting your scoring and HUD setting to competitive score so you can keep track of you time and score (with the time multiplier already applied).
After 15:00, you will notice your score start to drop. If you kill enemies fast enough you will earn more points than you're losing and can still hit 25k.
Catch - Grunts will throw more grenades. This got me killed a lot until I learned to play it safe and take these guys out fast.
Famine - Weapons dropped by AI will have half ammo. If you're precise with your carbine shots, this won't be an issue.
Fog - No motion tracker.
Iron - Dying restarts the level. Save and quit as soon as you die and resume the level to keep playing with points intact. Doing this will reset your killstreak, however, meaning you'll get less points per kill until you build it back up again!
Jacked - Can only enter ground vehicles by boarding. There are no ground vehicles on this level.
Masterblaster - Co-op skull that won't affect you.
That's Just...Wrong - Increases enemy awareness of player. Makes the camo basically useless.
Thunderstorm - Upgrades the rank of enemies. Heretics don't have ranks.
- Be comfortable with the level. It helps to know where heretic elites spawn so you can charge your plasma pistol before they even see you. Sometimes there will be door that won't open and let you proceed until all enemies are dead. Know where these are.
- Take your ally's beam rifle at the beginning to take out the heretic elites right way. Your squad should be able to help eliminate the grunts at the beginning.
- The hardest part of the level is the beginning before you get decent weapons. Acquire a carbine and a plasma pistol ASAP. You'll be running the entire level with these two weapons. Famine is on, so the elites will drop less ammo, but you'll have enough if you headshot all grunts and plasma pistol + headshot the elites. Sentinels take 3 shots to kill. All grunts in this level have needlers, but you can find Plasma pistols with 100 battery scattered on the ground throughout the level (usually next to a needler and some plasma grenades.)
- There are TONS of plasma grenades in this level, use them to take out heretics easily.
-Grunts are probably more dangerous in packs than the elites with their needlers and grenades. The reason I didn't use the anger skull was because the needles were relentless. Eliminate grunts first when you can.
- Have the par score or be very close to it before the banshee part. Skip this section of the level and make it the door that ends the level. It's to the right after you enter the banshee. There will be more enemies on that platform that can boost you to 25k if you just barely have it.
I was pretty sloppy on my playthrough and had to save and quit multiple times but I was still able to get 25,760 with a time of 16:33.
You do have to complete the level quickly, but approach each encounter tactically and don't try to rush in.
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.