Got over 66k points just by playing the level on Legendary.
Died 3 times and got no time bonus.
This means you should be able to get the required 25k just by going through the level on Heroic.
With my score, you would have to finish with a 1.52x time bonus on normal, so you could take over par time and get it on Normal, but I dunno how easy the par time is on this level.
A few tips
I'm not sure how many points phantoms give but during the Mammoth ride I sat on one of the rocket pods and blew up a few phantoms. I also shot at any I could once I had the tank.
When the 'unidentified covenant vessel' attacks the Mammoth, make sure you grab the Gauss Hog that is parked by the cliff to take out the Ghosts and Wraiths easier.
I'd advise grabbing a rocket launcher from the mammoth when you get to the point where it stops and you have to leave it. Save the Rockets for Knights. It would 1HKO them even on Legendary.
PHARREAL87 - Achievement wouldn't unlock with required score unless played on Legendary. So if you have issues, on lower difficulties, try it on Legendary.