This achievement description is a bit misleading. There's 2 ways you can approach this. 1st one you can create your own epic lenght venture and complete all levels by yourself. The last level will be one of the 4 available bosses in the game: Prime Erf, Madcap, Syren or Nox.
Upon beating the boss you'll be given 2 options, incapacitate the boss and extend your venture or kill it and end the venture. Since the achievement description states you must participate in a COMPLETED epic lenght venture I recommend you kill the boss and end the venture, that way I'm sure you'll get the achievement. That's how I got mine so I'm positive it'll work that way. Don't know if it works if you choose to incapacitate the boss though.
The other option you have to get this achievement is like the solution above describes. I'm just gonna complement mosin360's solution and clarify some aspects that may not be clear enough.
There's 5 types of ventures in this game: mini, short, medium, long and epic. The difference is the lenght of the venture. Mini is 5 levels long, short is 10, medium is 20, long is 40 and epic is 80. On the venture selection screen you'll want to look for a venture with 80 levels. To make your life easier select the easy venture feature and pick the one that is most advanced.
Now let's say the venture you picked has 43 levels completed. Once you hit play you'll start from the last community checkpoint. Now if you're not in a rush to get the achievement you can simply die on the spot and leave the venture. If someone eventually complete this venture you'll receive a reward for participating in that venture and you'll get the achievement or if you complete the remaining levels yourself you'll also get the achievement. Plus if someone complete the venture you created you'll also get the achievement case you started a level of your own venture, doesn't matter if you beat any level or just die on it as long as you participate in any venture that eventually gets completed the achievement will come.
just a heads up: after level 40 the difficulty is increased. You should be very skilled with your choosen character if you play this game long enough so no big deal. The only real difference I felt difficulty wise is the type of enemies you encounter. Regarding their behavior and damage delt I think it's all the same.
I hope I got everything cleared out for anyone who had any doubts about this one. Good luck to all!