Map: ONI: Sword Base
Difficulty Level: Legendary
If your already running your Solo Legendary Run just turn scoring on. Get through the the first base area first and take out the two enemy Wrath's. You should have a warthog a this point. Have Kat as your gunner and set up the Marine with the rocket launcher. The rocket launcher can be found on the hill (rock outcropping) to the right of where the Wraiths will stay when they attack. Now just drive your warthog around. Your NPC team will do all the damage while you obtain score. You should have about 30-35 thousand by the time you have to return to Sword Base which is well above the 25k required.
Extra info from Shadow 00 Fox‘s comments
You can also add, at the anti-air gun reactivation during the warthog point farm, you can farm points there if you need too, considering it's easy to die in the later parts of the mission--the phantom keeps respawning and your gunner and rocket passenger can take it out in about a minute, it's worth a lot of points, 1k at least!