Be-aware that as of February 2016 the online for this game is still popular with randoms and boosters alike so you may have some conflicts when you are trying to go for this achievement.
This is a very annoying achievement because unlike most of the others, it cannot be done if you are placed in a group with some other players/boosters. The reason is because the Infestation game mode will be faded out and you cannot access it unless you search individually.
The minimum amount of players needed to start a match towards this achievement is 4. But the problem with this game mode is that if everyone lives in different regions, it will be close to impossible to meet up with each other. So you will likely need at least 2 people in a boosting group to help you search for a match that live in the same region as yourself.
So once you have at least 3 people including yourself that are available to do this game mode, everyone needs to individually go into Ranked Match, Quick Match and then choose Infestation. When choosing the amount of people you wish to look for I recommend 8. Only choose 12 if you have a hard time finding each other (this may increase random threat).
Now the online matchmaking for this game is extremely difficult because when you search for matches, you will have no idea if you are searching with your boosting friends or not and you won't find out until you are on the lobby screen. You can however keep in contact with you and your boosting friends by saying how many people are on their screen when searching. If it's the same it will likely be you've all found each other.
Eventually after a short period of time the game will start counting down and reducing the amount of players need to start the match. Randoms can join your match still and they can do so until the match is loaded. When the game reached "0 Players Required" you will have a lobby. You have 30 Seconds then to choose your characters, vote on maps and your skins for the characters. For map-wise I recommend Refinery, The Gateway or The Jungle as they seem to be the smallest.
Now here is the important part. If everyone's ready they need to press the
button to ready up and then they will have a tick on the left side of their gamertag. If everyone readies up, the match will load instantly and the 30 second timer will stop. This will load up the match and you will also prevent random players from joining it from here on out.
When all of you are in the match you will all start off as Marines for 30 Seconds. After the time duration one of you will turn into an Alien. If you are the Alien at this point of the game, you cannot go for the achievement and you will either have to let someone else go for it, quit the match or finish it and get a bit of XP.
Whoever wants the achievement needs to avoid any Alien contact completely because if you die, you will void the achievement. If you are boosting you first need to communicate because anyone playing as a Alien will not know which Marine to attack or kill. Signal the player who is the Alien by telling them where you are, what skin character you are, if you are standing or crouching etc. Now with this in mind the player who is the Alien can go ahead and kill the other people that are not going for the achievement.
When all of them except for the last Marine are dead, another timer will begin in which that lonesome Marine must survive the duration. If they do they will win the match and get the achievement. If they don't then the whole match is a waste of time and another one must be done in order to get another opportunity.
Note: Discovered by my friend Spangles21, if you play as a Marine and commit suicide, you will respawn as an Alien.Once the player who wants the achievement gets it, rinse and repeat all of the above until everyone gets the achievement. As long as you don't get assigned as an infected at the start of the match and everyone co-operates with proper communication, this achievement is do-able.
Good Luck!