You can use Silver's ("Halo Completionist") guide for Monopolized (speed run in less than 3 hours) to pull this one off as well:
To start it, set the game on:
- Timer on
- Skulls: So Angry, Piñata and Grunt Birthday Party (all optional). Do NOT turn on any other skulls that may void this achievement.
At the beginning we won't do the 1st despawn of enemies. Instead, get on the Spectre, betray the Elite, park the Spectre, get invisible and hijack a Ghost. Pretty much the way he shows in his video, except we will rush all the way past the Scarab and fight the Brutes and destroy the 2 Wraiths at the door, so it will open. On Easy difficulty you shouldn't have problems taking down all of them without getting killed. To make things even easier, one option is to hijack the first Wraith you see on this level and use it to kill the Brutes and Wraiths at the door. Doing that sped things up a little bit more for me.
In the next sets of rooms follow the path showed in the video but do NOT take on all of the enemies. Use your cloaking/invisibility ability to get past the enemies as soon as the doors open. In some rooms you will have to kill a few Brutes in order for that to happen, but the focus isn't the fighting at all except when you get outside the building again, when you will have to kill the 4 Brutes as soon as possible in order to trigger the next cutscene.
After the cutscene, don't forget to do this next despawn (getting back inside the building until it says "Loading...done") as Silver does. That is essencial. Then proceed to the Spectre and follow the path that he shows. If the vehicle gets stuck at the top of the structure that you have to climb it's because you weren't pressing
to tilt the nose of the Spectre like he told you to do. Save and quit and then reload the game. It's the only way to try again, since trying to get the vehicle out of there with plasma grenades will destroy it.
The rest of the video is just like he shows. You can wait for all 3 Sgt. Johnsons to appear or move on with just 1 or 2 of them. Following this method you will finish the mission in part time on Easy.
If this solution helps you in any way, please give it a thumbs up to let me know. Thanks!