This achievement is fairly straightforward, just finish the game on any difficulty.
I did have an issue with this unlocking. After I finished the game I had to restart the game, load back into the menu and it only unlocked when I started a new game. Hopefully this doesn't happen to you guys, if it does give that method a go.
One other solution if it gets stuck is to go to the game hub (offical club) and tab over to the progress section.
Good luck :)