So after a week off for traveling, I just finished Halo 3 on Legendary ... sort of. That is, I've literally played through every single mission and level in the game on the Legendary difficulty and beaten the game. (I finished the last two missions today.) There's just one problem: I didn't get the achievement because the game glitched on mission 7 a few weeks ago, and I didn't realize it hadn't recorded my completing that mission until I was part way through mission 8. (You can go into the UI and see which missions you're credited for completing on whatever skill level.) My assumption is that I can just play through mission 7 again, all the way, and get the achievement, so I'll do that now.
It better happen that way. :) Damn you Bungie.