NOT SURE but with MORE people currently having this achievement it may have been properly fixed last patch so this solution may be for not now. I will leave it posted if you are still having problems getting this one.
This is just some advice about this one. The tracking system in this game is broken which is why so many achievements are screwed. I have been going for the War Board mission achievement for a week now (10 runs and counting). There was a patch a few days ago that seems to have fixed some achievements. I am 1 of 2 people who have this one, I was going for it as well while going for the war board one.
Anyway I THINK what helped me here was I have been un-installing and re-installing the game to try and clear mistakes up and have a start fresh. Now for my 10 run today I decided since the tracking is broken I would play continuously, never quitting the game, never going back to the game menu, because I think this is where the tracking losses its place when you leave the game. The other 9 runs I did I know I got the 25 missions (I counted) but it would not pop and I did leave the game and turn my xbox off those times. Today was a 16 hour marathon staying in the game never leaving (not even to game menu) and it popped at 25 missions. As for War Board the game screwed me AGAIN today and left a door LOCKED that should have UNLOCKED and I had no way of unlocking it again as the mission has progressed past that part, this was my very LAST mission too, would have been a waste of 16 hours save not for the 25 mission success today.
Best of luck with this, and this is only a theory I have as to why it worked this time around so try it at your own peril not GUARRANTEEING that this will work again.
If it does WORK for you leave a comment so others will know this theory works thanks. If it does not work for you also leave a comment and if I get 3 or 4 of those I will delete this post as I have NO IDEA why it worked for me today then.
Glenn 46