Breathedge

UPDATE 2: Based on feedback and comments in the Steam forums, it looks like the timer may reset after 24 hours, so the trick may be to slow roll and don't beat the final boss until after the gamesave timer hits 24:00 but before 31:00. Getting it to pop legit under 7 hours isn't too difficult, so if you are close to finishing under 20 hours, its probably faster just to finish and then make a second run.

UPDATE 1: And it didn't take long to get some feedback where it isn't popping even though the completion time was less than mine. I'll keep this solution up temporarily until we find some better logic as to why it pops for some people or until someone posts an actual solution with a speedrun guide/video.. The safe bet is to play under 7 hours, although that will require a separate playthrough since it's unlikely you could complete all other achievements at the same time.

This achievement supposedly requires the completion of the game in under 7 hours, but appears to be be buggy and will pop upon completion with much longer times. I finished the game in 26 hours 52 minutes according to the save slot timer and it popped immediately after the 'Wasted' achievement for completing the game. Looking at the Steam forums, it seems to be bugged in the same manner on Steam as well. If it doesn't pop for you, post your playtime in the comments so we can determine if there is an upper time limit or if it is unlocking inconsistently. I'll remove the solution if it gets patched.