Not really a unique solution, but now that the gaming sessions feature has been added to this site, obtaining "With Friends Like These" is much easier. Make a session for it, preferably a week or a few days in advance, and set for 8 players or more (or unlimited for that matter) needed. People seem eager to get this achievement, so you should be able to find people willing to help out.
If you'd like, you can add other TF2 achievements to the list (such as Flamethrower or Turret), to broaden the chances that you'll find people interested. Just be certain that everyone in the session has added each other before the match starts, and you probably will all want to be in a party to communicate easier.
As pointed out before, you need to complete a full round of the game type for the achievement to unlock (i.e. One team must capture the flag 3 times to end the round at 2Fort). As long as no one leaves the match mid-game, everything should go fine.
Good luck, and have fun!