EDIT: According to TheDarKnyht, as of the October 2011 update Stockpile has been returned to the Team Objective Gametype and will now hopefully be easier. Good Luck! :)
This achievement was made difficult by Bungies removal of the stockpile playlist and the removal of the gametype, from DLC objective and team objective as well as flagstravaganza.
This gametype can only be found in Big Team Battle and Team SWAT (as of 8 July 2011) may change over time. This achievement CAN'T be unlocked in speedpile in action sack, it must be the original gametype.
You have to capture all the flags on the map at once, the capture zones reset every 60 seconds I believe so you need to bring all the flags into the circle within that time for them to all be captured at the same time. Any member of you team can bring them into the circle and the whole team will get the achievement.
To do Legit- Enter Big team Battle or Swat with a team or if you want to try and wing it. co-operate and keep all the flags near your base, steal if you have to form the other team's capture zone.
To boost- The stockpile gametype is very rare now. It only appears in Big Team Battle and Team SWAT. So to boost gather a team of 8 players (you can sign in guests, however they must get a few kills other wise you will banned for AFK boosting) play Team Swat and vote none of the above in an attempt to get SWAT STOCKPILE if you don't get it, play the game normally assuring all controllers get kills, then hope for it next voting round.
When you finally get SWAT Stockpile talk accross xbox live party to have one group run all the flags into their capture zone, other team does nothing. Flags capture then the other team moves all 4 flags into their zone and gets the achievement.