This achievement is missable and you will either need to return to an older save or start a new game to earn it. The stats page may have an incorrect completed mission count when doing the Loyalty missions, but if they do appear in your 'Completed' log, then it's okay to continue to the end.
This can be unlocked in co-op. Thanks to Tasty Outlaw for confirming.You do not need to finish side-missions (store hacking/activities) given to you from your crew
, but doing them will progress towards other achievements while you're working through the game. Thanks to Mjilner and Cr4zyAlan for confirming this.
Before the final mission, you will get a warning prompt that once you start, you will not be able to return to the simulation until it is over. You'll need to finish every Loyalty mission before you start the finale. You can save any of these Loyalty missions until just before the end-game mission and clear them up one after another if you'd rather focus on the story/activities first.
Final mission spoiler:
You will get this before Imperator
during the cutscene after the credits.
Disclaimer: I taken zero credit in this guide, I am merely copying the american versions guides for people with the Australian version because it is more convenient for them.