Getting a par score of 25,000 on the Ark is doable on normal setting without even using any skulls. It is probably the easiest of the Halo 3 levels to obtain the par score on.
You'll probably need a little bit of a time multiplier to boost your score up. You don't need to rush past groups of enemies, but you'll want to wipe them out quickly. The level is quite long and the par time only 20 minutes. After par time, your multiplier will begin to drop steadily until it levels out at 1x.
Getting kills without dying builds up your killstreak and earns you more points. Multikills also give you slightly more points.
Setting your scoring and hud to competitive score will allow you to see during the game what your score is with the time multiplier already applied. You'll notice after 20:00, the score will start dropping, but you should have more than enough to end up with 25k by the end of the level.
When playing the level, you'll want to grab a chopper as soon as they're available. They're fast and have got a lot of firepower.
Be sure to destroy all enemy vehicles, even if they are not occupied. You will get points for this.
Once you are in a tank, try to kill any phantoms that are hovering and dropping off enemies. They're worth 800 points each.
When the scarab shows up, a marine will stop by you with a gauss warthog. Hop in the driver seat and let him shoot. Alternatively, you can take one of the scorpion tanks. The heavier firepower will save you time in the fight with the scarab, but beware of the scarab beams, as you'll be an easy target.
Be quick and thorough when navigating the interior of the forerunner structure.
Try to have a precision weapon to take out the brutes at the end. Watch out for the chieftain's invulnerability.
I ended the level with a score of ~54,000 and finished in a time of ~25 minutes.
WATCH THE FULL CUTSCENE AT THE END OR THE ACHIEVEMENT MAY NOT POP.
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.