Iron Brigade

To get this achievement, you need to complete a mission in multilayer with three other people.

To play with other people, you have to go to the radio room, and recruit other marines. The radio room is located opposite to the mission briefing area. If you haven't completed the first few levels in solo, the radio room might be inaccessible.

Once you are in a full ship(lobby), I would advise you to choose the Pylon mission. The pylon mission can be completed within a minute, so it will put down the chances of someone quitting out of the game. The Pylon mission is the final Europe level, and the first boss level.

This is an easy achievement to snatch up. You first go to radio room and search for marines. This go's alot quicker if your by yourself, so I recommend searching by yourself. After you get a full lobby of four people including yourself you then should pick a quick boss level (Pylon, First European Boss) or what the group decides on. You then just have to complete the level start to finish. Then your achievement unlocked!

Credit to the other solutions for the mission Pylon. I just wanted to add that I had difficulty as people kept choosing Farnsworth and someone would be disconnected pretty quickly due to the network connection. What I did was recruit until you have a full lobby. Go and choose Pylon. Then go on mission selection and sit in this screen (a little shady yes but you will get all these ignorant players the achievement) when the selfish little buggers come along to try and change the level (have had times when we kept swapping back and forth lol) it will say the console is already in use and won't allow them access. Although yes it does run the risk of them quitting out and you won't know, it is the best method and gained me it first time as took down the boss in 32 seconds.

Good luck!