Here's the topic where I found out about this. Apparently 343 tried to fix it, but messed it up even more to make this possible. If you want to do it, do it fast. I'm guessing this will be patched soon. I'll go to bed now, and when I wake up I'll do the last 2 or 3 missions I have left and let you know if I got the achievement. I'm pretty sure I'll get it achievement.