Considering that the other two solutions did not fully explain how to set up all of the details for this achievement, I decided to create one explaining exactly how I got it. I tried knight0fkh0nshu's solution above and it did not work. While I did follow his idea of Co-Op Elimination with infinite spawns on, I did not get the achievement after waiting for over nine hours. So I tried again but with the settings listed below.
Several comments were wondering about such things as number of controllers,being connected to XBOX Live, and controllers turning off, so I will explain all of these things. So, this is how I got the achievement:
From the main menu, go to the multiplayer menu, then to local play. Have two controllers join the Identity Setup screen. I had the profile that I wanted to get the achievement join in the first slot (I do not know if it matters what slot they are in in the Identity Setup screen but that is how I got the achievement). The second controller does not have to be signed in to a profile, just joined to play the game (turn on the controller and join as the second player on the Identity Setup screen).
Next, for the Mode, choose Co-Op Elimination.
For Type, choose Create Custom Type. For Respawns, choose Infinite. I do not think the Enemy Density matters, but I had mine set to High. Hit the A Button to save the Custom Game Type. The slot you choose to save it in does not matter.
I doubt that the Map matters, but I chose Desert Gulch.
For Time, choose 60 Minutes.
You can leave the Kit settings alone, I had mine set to No Restrictions.
For Advanced Settings: the Number of Players should be 2.
Method for Player Assignment: Player Control
Team Start Location: Standard
The Team Camos do not matter.
Set the Auto Launch Timer to its lowest possible positive setting, which is 30 seconds.
I had my Game Start Timer set to 10 seconds, but I do not think it matters.
I had my Force Respawn Timer set to 10 seconds, but I also doubt that it matters.
I hade my Respawn Wait Timer set to Off.
I had my Respawn Invulnerability set to Yes.
Force Camera: Off
Kill Camera: Yes
Drones: 60 seconds
ID Friendly/Foe: Friendlies and Enemies
Lock Room: No
Class Modifiers: Yes
Show Enemies on Intel Map: Yes
Allow Weapon Pickup: Yes
Map Rotation: Same
Map Plays: 1
Voice Channel: Everyone
Allow Night Vision: Yes
After you have set the settings, hit the A Button to accept the currently applied settings.
You do not have to worry about any of the Player Options.
Now you can just wait for the Auto Launch Timer to start the match. After the match started, I disconnected both of my controllers and turned off my T.V. I was not connected to XBOX Live before or during the game (in case getting disconnected would not allow me to get the achievement). Let the game continue to play through 8 matches (because the matches are set to one hour each). I went to sleep and woke up to some easy Gamerscore.
I also did this after I had already downloaded the Title Update, for those of you who are wondering.
I know that some of the Advanced Settings did not need to be listed, but I decided to just go ahead and list all of them, to help limit confusion. I would rather be too specific than not specific enough. If you have any questions either leave a comment or PM me.