At the time of writing this solution, the "Domination" online game mode is pretty much completely dead. You get the occasional random joining every 5 minutes or so but it I can almost guarantee that you will never be able to get a game of "Domination" started by just waiting in the lobby for 5 other people, and you need to do this multiple times to be able to play 12+ games of Domination to hack all points.
So what do you do?
1- Play a couple games of TDM and then search for a game of Domination. Proceed to spam party and game invites to all of the people on your recent players list and hope you can amass a group of 5 other cooperative randoms (not an easy task)
2- Try and get people from TA to help you boost this. This is also a surprisingly difficult task since you need to convince 5 other people to go on a relatively old arcade game such as this and spend 3+ hours for a single achievement.
A combination of both is probably the way to go. Get a few people from TA who still play this game and play one or two games of TDM. Then try to get enough randoms to help you start games of Domination. If all goes well and nobody leaves or d/ces, **the game will go through all 12 maps in a row**(meaning you won't get the same map twice and the maps won't be randomly generated), making this achievement obtainable within 2 and a half hours. Not the easiest thing, but possible.
If you need help with this, I would be willing to do it. Send me a PM via here or XBL.