This is best done on the 1 vs 1 map Chasms. Use the Arbiter for this one. Set the AI to Legendary, of course, and have Captain Cutter be your opponent.
As soon as the game starts, build a temple and three supply pads. Scout with your Ghost for supplies, there are a lot near your base, and about 60 to 90 supplies in the center; collect those with your Ghost and move the Arbiter to the center of the map and wait. While you wait you can build a fourth supply pad, there is no need to upgrade to a Citadel.
By the time the Sentinel spawns (the flag), you can almost instantly grab it with Arby. Grab it and go camp at the back of your base, and wait. You will notice a timer running, that is how long it will actually take for capture the flag completely and have a point added to your score. You need three points to win.
Upgrade one of your supply pads every now and then, in case Arby needs to get into rage mode, but personally I never had to do this.
Now, when the timer has passed and you are rewarded a point, go back to the middle with Arby. Give him the upgrades ''Fiendish Return'' and '"Defiant Rage'' if needed, found at your Temple, and wait for the Sentinel and capture it, again camping at the back of your base. Repeat one more time.
Cutter will almost never send his units to you, he will be too busy building up his base. But if you really really want to be sure, listen to the AI about what kind of units Cutter makes, and build two turrets at the front to counter appropriately.
You will be rewarded the achievement after three Sentinel captures.