Figured I'd add a new quick guide for this for the new Reach or 2020 update to the MCC that no longer lets you do 2v2(So 2 people, each with an extra controller)on Halo CE to get this achievement.
If you try Halo CE 2v2 on Zone Control it'll say "The games included and game category combination is invalid."
Since my recent session for this was going to be trying that, it was annoying to be sure. But I and my boosting partner quickly found a new method, that could possibly be even faster then the old Halo CE one.
Pick Halo 4 2v2 on Zone Control. If you played or boosted the Gears of War series, you might know how this works. You'll be doing ring boosting as it were. For the first 5 seconds you are in the hill, you'll get the Loitering medal. So you and your partner will just take turns holding the hill for 5 seconds at a time. Just keep switching back and forth, following the hill as it does move every minute. Plus since it's Halo 4, you'll always be able to sprint, which was nice.
You can also get a medal for Hill Defense, killing a player while holding the hill. When I started I was around 35% progress into The True King, my partner was at 10%. So while ring boosting, I would bring my second controller next to the hill, so my partner could kill it, and get extra Hill Defense medals. You'll get about 10-11% towards The True King each match from the Loitering medals, and an extra 2-4% for Hill Defense.
Overall it took only about 1 hour and 15 minutes for both of us to finish. So if you are starting at 0% for The True King, it should only take you around 1 hour 30 minutes to maybe 2 hours to finish. Depending if you are getting extra medals from Hill Defense.