Guys, I found a way that personally fixed this glitch for me..
So I completed all of Realistic a year ago and didn't get Freedom Shall Prevail.
I got the game back last week, and planned on playing through Realistic with a mate. I played the first mission up until about half way through, where the Ambush checkpoint began, and it popped. So try that out if the achieve never popped for you, it worked for me.