This HAS to be done on a Defiant map. Furthermore, this can obviously not be a multi-flag game type. For those who don't know, the Defiant maps are Highlands and Condemned and any of their variants (e.g. Uplink). The opposing team cannot even pick the flag up for this achievement to pop.
Update: When boosting, it is highly recommend neither team touch either flag for this to unlock. Several players have reported even though their flag was not touched the achievement did not pop because they were able to grab the opposing teams flag.
MAKE SURE you are on a Defiant map. If you are having issues getting defiant maps remember that everyone in a lobby has the have that map pack in order to even have those maps come up. I'd suggest going to DLC specific playlists to have a higher chance of getting Defiant maps.
Lastly, you need to have the Anniversary pack, whether it is by purchasing Halo CE: Anniversary or purchasing the stand alone Anniversary Pack.
Update: For boosting purposes it is best to have someone remove the Noble Map Pack so that the only maps offered are Defiant and Anniversary. Team Objective is your best bet to get a 1 flag CTF game on a Defiant map.
Update to the update: Premium battle has been removed as of 12/5 so Team Objective is probably the best bet.
Update mark III: It looks like there are more playlist changes coming for January 2012.http://halo.xbox.com/blogs/Headlines/post/2012/01/04/The-Hal...
Not sure if this will change the odds of finding Condemned or not, but they go live 1/10/12.