Rabid CompletionistG 120

On Difficult Difficulty, you need to finish all the career missions, side missions, Action Zone, and Bonus missions in order to earn this achievement. In order to see your progress, from the main menu, select 'Extras' and then select 'Statistics' and you will see what you need to do to finish 100%.

There are 10 career missions, 9 side-missions, 5 bonus missions, and 24 total action zone missions.

You also need to collect all 7 weapons, but if you manage all the career and side-missions, this will come naturally.

Good luck and have fun!