Boosting all of the multiplayer achievements should take about 30 hours or less. There are several achievements that ask you to get a total of 40,000 points in ranked matches, and then there are a few for doing some specific tasks during a match. Some of the miscellaneous ones can distract from point boosting, so your session will have to cooperate and allow you to earn those achievements.
There are some multiplayer maps that can be downloaded, if you're interested. One is free and two are $5 USD each; the ones that aren't free are not worth your money whatsoever, and the one that is free isn't worth your time.
The key set of the online achievements are based around earning 40,000 points in ranked multiplayer matches. You can check your current standing with the in-game leaderboards. Points are earned for killing teammates and finishing objectives, but points are also lost by killing teammates. The only way you're going to get these achievements these days is by boosting, as no booster will ever want to play real matches if they can spend their time better by just boosting their points.
The accepted boosting method requires eight people minimum, as a game cannot start without this minimum, but there's no harm in having more people. The game can be double-boxed through either two 360's, two One's, or one of each if you're just below having eight people. Having boosters from faraway locations (varying countries, etc) can greatly accelerate the process, as creating a decent amount of lag can improve your scores.
Someone will have to choose a Capture the Flag match through the Custom Match tab to select the game mode. When in the lobby, the person widely accepted to be in the second position in the list is actually the host of the match; if the host isn't from a country farther away from most of the other boosters, have the current host quit and rejoin until the desired booster is the host.
Note: too much lag obviously makes the match unplayable or unjoinable. Simply having a fair/strong connection outside of the most common country between the session is totally fine. Even then, having this lag isn't a requirement; it just helps with point boosting.
Note: It is not necessary to hear the game while boosting. The game audio during boosting is going to drive you crazy if you have a 2-3+ hour session going. Try to turn it off or lower it if you can, but make sure you can still talk to your party. Try to get some kind of video or music playing or something, but not at a volume high enough that it disrupts your fellow boosters.
Once a match has been created, three Allies and three Axis will stay at their flag, crouching a small distance away. One of the members of each team must go to the enemy flag to act as flag-throwers. The flag-throwers should switch to their secondary weapon, as that makes the throwing process a bit faster. The flag should be thrown into the mass of three enemy boosters in a way that ensures the flag lands in the middle of them so they all get credit for it. With any amount of decent lag, the crouching trio should earn more than 1pt per flag return. The crouching team should frequently - almost always - be checking their leaderboard to see if they're actually getting points for being within the pickup distance. If not, rearrange yourself to continue earning points. The game does not kick players for idling.
The flag-thrower does not need to hold to pick up the flag. They can simply tap the button. If the game forces you to switch to your main weapon out when throwing, shoot out all of your ammo. The game won't switch you to your primary weapon if it's empty.
The one caveat to this method is that the flag-thrower will only get 1pt per flag pickup, no matter what. It's also far more tedious to perform than crouching and simply making sure you're getting points. As a result, nobody should reasonably want to volunteer to be the flag-thrower unless they feel particularly generous that session. Another problem is that, after a few matches, you're bound to have multiple people on one team that have already thrown the flag, making the balancing of this duty to almost always be uneven. It's the hosts' responsibility to keep track of who has thrown and decide who will be throwing in the next game.
Make sure you're aware of the miscellaneous achievements at the bottom of this page, as some can be knocked out in a session without forcing everyone to help out.
Colonel (20000) tends to glitch but the general consensus is that if it does, it'll unlock in the 20,000 range. If not, then you'll technically have to kill teammates to force your score down below 20,000 again. This is a big problem if you're doing this after scoring the last achievement, but there have not been any reports of anyone having to do this.
The easiest of the other multiplayer achievements to earn is Supply Officer, which asks you to give twenty ammo supplies to teammates in one match. Have a teammate fire at the sky while your class is set to Support (so you can throw the supply drops). When boosting for points with the flag-throwing method, ask a teammate to fire at the ground or in the air - just make sure they're not hitting anyone. Then drop your teammate some supplies. Since they're losing ammo, they'll be able to accept ammo from the supply drops. You should get your achievement after your ally has picked up twenty of your drops.
It's also fairly easy to revive ten teammates without demanding valuable boosting time from your group. First, of course, choose the appropriate Medic class. When you're sitting by the flag with three of your teammates, have the enemy flag-thrower shoot one of your allies, then revive them. One of your allies can also kill the other for this to work. Simply make them do this ten times for the achievement. If you kill a teammate in that match, you will be unable to obtain the achievement, so be careful.
The last set of miscellaneous achievements can be obtained at the same time, but they require you to play a different game mode and the cooperation of your boosting gang. Set up a game of War (not Capture the Flag) where one person does all the work in a match to capture all of the required positions until the game ends; that way the person doing the work is sure to get the achievement. If the person doing this makes sure they have the most points in the match (out of both teams, just to be safe), they'll also earn Victory Medal. As such, nobody else but the person aiming for the achievements should be killing people on the enemy team; this will prevent anyone from getting close enough to mess things up.
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