I would like to add more information in addition to the previous guide by Ehrenfried. So here is my suggestion for those who don't have online friends to boost with.
- Attempt to join random campaign match
- If you success in joining, you will be immediately directed to a screen where you will see names of other players (on the right hand side). There are four rows, each player’s gamertag will be displayed in each row.
- if you see that all the rows has gamertag (that means there are four people including you), you are good to go. Try to finish the level and hope that nobody quitting out. Or if someone quit out, but new person join in to complete the group of four, you are still good. Complete the level for your achievement.
- However, if you see that there are less than four people (you see one or two blank rows without gamertag), you may drop out immediately or wait a little bit in case someone join in and complete a group of four.
Again the idea is to attempt to join a game that already has 3 people playing. It does not matter if you start from the lobby or in the middle of the game, as long as you are the fourth person to join, you will get the achievement. The other three who join earlier before you will not get the achievement.