This achievement is not for completing all the story levels. It is for finishing "The Final Dimension". You can get it without completing a single story level.
If you join a co-op game before the end of "The Final Dimension" you will get this achievement, but unless you joined at the start of the level you will not get:
At the time of writing 4,432 tracked gamers have this achievement, and yet only 4,230 have "A Serious Loophole".