EDIT: Thanks to TheDarKnyht for this information - "To everybody still hunting this achievement, Stockpile as of Oct 4th 2011 is back on the playlist for Team Objective. I managed to get this on the first try thanks to two people dropping from the other team."
This is much easier to do boosting, but all you have to do is collect all the flags on the map during one "round." Most maps have four flags, I believe. So basically, you have to capture the four flags at the same time. If you don't want to boost, get a good team together and communicate! You need people to get flags (I recommend two people working together, one to carry and one to defend) and people to defend flags once you get them back to your capture location. I suggest you place all the flags outside the capture location in case you do not get all four (or however many are on the map) before the countdown hits zero and starts over.
It is a team effort, so everyone can capture the flags. Once all four are capture, the entire team will unlock the achievement.
This achievement is NOT to capture all the flags that spawn on the map for the entire game of Stockpile (aka you do not need to win a game 10 to 0).