Halo: Reach

Download this gametype, and play with a second controller or a friend in a custom game:
http://www.bungie.net/Stats/Reach/FileDetails.aspx?fid=15148...

If you are having trouble with the above link, download the gametype from the following webspage:
http://halo.xbox.com/en-us/haloreach/PancakesLP/fileshare#%2...

Play the downloaded gametype, on Countdown, and meet at the base of the person getting the achievement. Have the other person take the flag, and then put it down. After standing near it for 5 seconds, it should respawn to its original place. Do it again, and the achievement should unlock in-game.

Note: It will only work if you play this gametype (download), and NOT regular CTF that the game comes with.

Update: As of August 19, 2012, this method still works

When this DLC released, Bungie created special achievement game types on their website to make this easy to boost. Some people have trouble downloading these from Bungie.net, so I put them on my File Share along with a custom map to make things go quicker.

From the menu, select File Browser, New Custom Search, File Share Search, then enter my gamertag (Demonex Machina). Download "2 Flag Pro" and the map "Defiant DLC Achievements" (For all three Defiant Map Pack achievements, download everything).

With a second controller or a boosting partner, go to Custom Game, set the game type and map (The map is a Countdown variant) to the ones you downloaded and start the game.

Have your second controller or boosting partner grab the flag and toss it to you. Stand near the Flag for a few second until it returns. Repeat once more for your achievement.

You CAN win all of the defiant dlc achievements in CUSTOM GAMES!
Simply download the '2 Flag Pro' gametype to your xbox from bnet and load it up on reach:

http://halo.xbox.com/en-us/haloreach/PancakesLP/fileshare#%2...

[This gametype will work whereas the in-built CTF gametype will not]
So grab an extra controller or a mate and have at it =D (Thanks Joda X & Maka91!)
All credit for this youtube video demo goes to OpTicNiko.
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[!] You must return two flags before the end of the game [or the end of your last defensive round in 1 Flag CTF] however you do NOT have to return two flags in a row [but you can].
eg. you may return the flag, then your teammate returns the flag, then you return it a second time and you will still get the cheevo.
[!] This does NOT have to be done on a defiant map [but can be].

This is how you do it legit for if/when they patch the custom games...
|> Load up team objective [preferably] or BTB and vote for a CTF game variant
{All three of the defiant dlc achievements can be won in the team objective playlist}

- Once in a matchmaking Capture The Flag game, simply wait for an enemy to take your team's flag [you will hear "flag STOLEN!"], kill them and recover the flag by standing beside it.
- A reverse progress bar will appear at the bottom middle of the screen in your team's colour. Once it has fully drained, "flag returned" will sound and the flag will be returned to your base.
Easy.
- Now rinse and repeat and pop goes the cheevo :)

NOTE:
~ To receive credit for returning a flag you simply must be close to it at the very last second before it is returned, you do not need to be beside it the entire time the progress bar is draining.
~ Multiple players may return the flag at once and all of the said players will receive the credit for the flag return.
~ If engaging at long range you will know when you kill the flag carrier as "flag dropped" will sound and you will receive a killed flag carrier medal which is a blue flag with a red dot on it in a gold octagon with a white background.
~ In CTF a smart enemy trying to score will either get a lift in a vehicle driven by one of their teammates back to their base or use your base's teleporter and then get a friendly lift or walk it back. Preventing this is key or else the flag will quickly be out of effective range and difficult to return.
~ The "flag TAKEN" sound cue occurs when YOUR team steals the ENEMY flag. "flag STOLEN" sounds when the ENEMY team steals YOUR team's flag [this is what you want to hear].
~ Camping near the flag with a shotgun is a very effective way by which to kill the flag carrier, but beware of the flag carrier's melee instakill.

Hope this helps
Happy gaming

coreymawf

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All credit for this youtube video demo goes to OnyxMovies117.

For this achievement you don't actually have to have the DLC or any other DLC's.

You have to return 2 flags that have been taken from your base in matchmaking or custom games, you can just have a guest acc. or booster pick up the flag and drop and then return, and rinse and repeat, i suggest using sword base.

This is the gametype for the achievement in custom games.
http://www.bungie.net/Stats/Reach/FileDetails.aspx?fid=15148...

To put all of what people have said in short:
Your flag must be out of its area, and you yourself must personally reset it. But more than one person can reset it.
If you are unsure whether you are reseting the flag or not, it should bring up a bar on your screen indicating the length of time left until it resets.
When the voice over says "Flag reset" this is what is requires
you have to do this twice for the achievement

If you want this one without custom maps, the best gametype I've found for this is 1-Flag CTF. This way, you only need to worry about capturing or stealing the flag in your respective rounds. The only real problem, depending on your opponents' skills, will be either catching them once they have the flag... or letting them get the flag in the first place.

Pinnacle was the map I got the achievement on, and it was somewhat easy to camp around the flag to make sure they didn't get far. Other maps, of course, have their own advantages and disadvantages.
Just keep it up and make sure you stay alive long enough to capture the flag each time, and you should get this one in no time.