Thank you for everyone's comments, Maka91 cleared up the issue in a comment
" If you press X at the mission select, and start the game on a new save slot under Veteran, you'll only need to complete 1 ending (Ashes to Ashes is easier). If you level select your way through a previous save, you'll need to do both ending if you have them both unlocked."
Full credit goes to him on this note, which also makes sense since I did Veteran on my first run through and just went back and did level select through the menu.
Thank you Maka91, never though I would ever get a comment on a solution from someone I watch and admire.
Definitely a high point for me as an achievement hunter even if it's a basic guide on a pretty straightforward achievement.
Some have mentioned in the comments that they didn't need to complete Ashes to Ashes if they didn't chose that ending. For me I did then it popped so your mileage may vary
I would like to echo the other guides in saying it is to beat the campaign on Veteran.
But in order for the achievement to pop you must complete both missions, Ashes to Ashes and The Final Countdown. The choice for the different missions to either
is found in the mission Identity Crisis.
Another side not is you DO NOT need to complete the side missions for this to unlock. I had only completed one (Red Circus) during my Veteran play through and still got the pop.
Hope this helps, Happy Hunting.