Marvel vs. Capcom 3: Fate of Two Worlds

Just like Hokiie said, create a private lobby, then invite your mate in. Get your mate to start at the bottom of the list and you start at the top. Work your way down while he works his way up. Once you get to the bottom and your mate gets to the top, choose the last character you haven't played with and start. As soon as you start fighting, BLEEP BLOOP!

*You can leave and come back, it doesn't need to be done in one session. Also, have all the characters unlocked (which means you have to get 8,000 PP, I think).

If you are going to give me negative votes, at least leave a comment on what I did wrong.

The way I got this achievement was by creating a private lobby and invited my friend in there. Then when it went to the character select screen we chose random everytime. It may take a while to get but if that method doesn't work you can always just make a list of the characters and check them off. You will know you are close to the achievement by going to your license and on the bottom right there will be a message saying "Full roster will unlock soon." or something like that.
Other achievements you can get while going for this one.
-Avengers Assemble (Captain America, Thor, and Iron Man)
-Darkstalkers (Morrigan, Felicia, and Hsien-Ko)
-Weapon X (Wolverine, X-23, and Deadpool)
-Female Flyers (Phoenix, Storm, and Morrigan)
-Fate of Two Worlds (Ryu vs Wolverine)
-Copy This (Spiderman vs Taskmaster)
-Raccoon City Incident (Chris vs Wesker)
-Fate of the Satsui no Hadou (Ryu vs Akuma)
-Brave New World (Just play one game online)
-Passport to Beatdown Country (Play on each stage)
-Badds to the Bone (M.O.D.O.K, Iron Man, and Spencer)
-Whose Side Are You On? (Captain America vs Iron Man)
-Duty and Deus Ex Machina (Captain America vs M.O.D.O.K)
And any of the misc. achievements. Ex: 100 blocks, 30 Advanced Guards, etc. Hope this helps.