Guys and Girls, thanks to BULLETPROOF and GEZDAGR8's hard work!

we have found a loading glitch that works 100% every time!
even with the NXE!! but has to be done offline! no need to try and battle
through them last checkpoints on veteran!! just do this glitch and walk through the game on Recruit!!! YES RECRUIT!!!!!

1. Go to Mission Select and select a mission on Veteran.
2. Once the level loads, save & quit.
3. Go back to Mission Select and choose the SAME mission, however change the difficulty to Recruit.
4. While the level is loading, and you are watching the cutscene, return to the Xbox Dashboard.
5. Reload the game and select Resume.
6. You should now be playing the level on Veteran with Recruit AI.

And all achievements unlock in legit order!