This can be boosted in a couple sessions with another partner (if double-boxing it then it would probably take overnight).
Island Capture, Swirl, Medium size, disable time and capture point limit.
When the match starts the person going for the achievement (player A) will build all rocket ships (LSMs - Rocket) while the other player (player B) will build all battleships (BBs).
Player A will split the rocket ships into three groups and each group will position around player B's shipyard. The rocket ships should be on average 3.1-3.7km away from the spawned battleships. Give it a little leeway and avoid hitting less than 3km as the battleships will be within range to hit the LSMs. Position the LSMs in such a way that it can effectively see 2-3 battleships. The AI should eventually be able to take out the battleships and shouldn't really require anymore interactions from the player (I had mine in the points screen so I know how much I need to get to a certain level).
Now, player B will have to constantly reproduce new battleships once they are destroyed. It can be time-consuming but if player B have a turbo controller (like Horipad) then you can set it to the support manager screen, pick the BB, and then turbo the A button and that should keep producing new BBs as soon as they're destroyed with no intervention from B.
Ideally you should end the match (other team surrendering) when your profile hit 2mil total xp as otherwise you could skip past the previous rank achievement (Lieutenant rank) and forever unable to unlock it (this is not confirmed but better off not risking it - there are also reports that mp profile is saved not on your gamertag profile but in save game, so if does indeed glitch you can potentially clear your save game and start over again). Restart the match again and once you hit 8mil total xp player B can surrender.
As mentioned above you will get this while boosting for this achievement:
Thanks to https://www.trueachievements.com/Darken81.htm
for helping me out and posters from http://www.xbox360achievements.org/forum/showthread.php?t=19...