Using the two controller method: i.e. doing it all by yourself.
First let me explain the 2 controller method, as I did not know how to do this when I first started.
Press start.... enter Verses modes enter X-Box Live Player Match (does not need to be ranked) Now press the Y button to sign into a guest account and then host a Private Match.
Set up: ANNEX, rounds 1, score limit 120pt. (NOTE* If you are going to start working on, or have already started working on the achievement THIS IS ANNEX! make your rounds 3 and this will count for that achivement as well.
For this example: I will use the Cogs (i.e.; Marcus, Baird, Cole) as the main account going for the achievement and Locust
(i.e.; RAMM, Bolters, Grenadier)
Take your guest, (the account not needing the achievement, the Locust) and have them win the first capture point. This should put the Locusts at 60pts.
Now comes the part I found most confusing.
Have that same account that just won, (the Locust with the 60pt. lead) and move to the next capture point. Enter and begin to capture it. When the Locust starts gaining points quickly move your other account (the one needing the achievement, The Cogs) to the same capture point that is being scored on. If it's a larger map or you’re unfamiliar with the map wait to begin capturing it until you have both (Locust & Cogs) accounts close to it, so you don't accidently screw it up.
When you are comfortable that you know that you can get both accounts to the second capture point move on to the next step.
Move the Locust out of the circle (capture point) or distance kill it, but do not enter the capture point until the score for the Locust reaches between 116 to 119, otherwise it will stop scoring points. The scoring will continue even if the Locust has been killed or moved out of the circle. So with that in mind, be aware this is a very close margin of error. You must now wait until the timer is down to between 4-3 seconds before entering the Capture circle. (Remember you have to win by less than 5, so you have to be fewer than 4 or less.) The good thing is that the moment you cross into the capture point the scoring immediately stops. BEWARE, ENTERING THE CAPTURE POINT WITH THE ENEMY (LOCUST) STILL IN THE CAPTURE POINT WILL NOT STOP THE SCORING.
Now finish capturing the last point, with the Cog, which should only have 4pts. or less. When that one is captured go find and capture the remaining two points with the Cogs for the win.
Now just repeat this for 20 rounds, not 20 matches.
Hope this helps..