Things to keep in mind like other poster mention: If you are playing and got booted whenever you would like to save your game, use a memory stick as storage, apparently it has some roblems save in hard drive while Co-Op play. II)Download the contents again and try to accept invite. I) Make sure you delete all the downloadable content (including the free 'Ninja/Psyco, etc) If you receive error message while try playing Co-Op Once another player join in, restart history and play through the game normally. Host - send invite to game like what you regular does. Until the time that Capcom has some fix install via next update. The achievement itself is not hard to get (Gameplay wise), What makes it differecult is the tech glitches that make gamers hard to join and play a co-Op game successfully.