To unlock this achievement you need to join a game and be the imposter. Only luck can do this for you!
You need to join any game, public or private. Note that you may not get the achievement if you are the host. You need to win via any method. If you accidentally use a vent, you will immediately void this achievement even if you pop straight out of the vent again. Try again next time!
If you are really struggling to get this through legit play, you can use multiple platforms to boost it yourself/with other players. You will need an additional 3 devices (pc/phone/tablet)- a total of 4 players - to launch the game. Make sure each device has its own account, you cannot join the same game if a linked account is already in it, and also ensure that the account you wish to unlock the achievement is not hosting. Any map will work but ideally not airship (players spawn in multiple locations).
When the game launches, kill the others. If the desired account is not imposter, call a vote and eject imposter to win. Try again.