I did every single level on Legendary except for the last on ODST and I did that in Co-Op with a buddy. We completed it and I didn't get the achievement and had to do the whole level over again by myself in Legendary...
When I die please bury me deep!
Place an MA5 down by my feet!
Don't cry for me, don't shed no tear!
Just pack my box with PT gear!
Cuz one early morning 'bout zero-five!
The ground will rumble, there'll be lightning in the sky!
Don't you worry, don't come undone
It's just my ghost on a PT run!