i did the first mission and the first two scenes of the second on my own, but then a mate joined MY game and we played until we had completed up to act 3 scene 2.
I then went offline and did up to act 4 scene 2 by myself, quit and did the first two scenes of the last mission by myself. the next day i completed act 4 and 5, then i hosted a game and me and a m8 completed the last scene of the last mission, then completed the sixth mission on my host.
i have ticks for all scenes on realstic but no achievement
i know its a lot of detail but i just wanted to make it easier to find out why it hasnt unlocked.
EDIT:got it today!
had a mate host a game, act 1 scene 1 on realistic (duh) and got the achievement at the outfitting box at the start of scene 2! yay