Straight forward, complete Reach's campaign on legendary, solo or with friends.
Here's a solo legendary playlist guide by Halo Completionist:https://www.youtube.com/playlist?list=PLsFiTv0qn72_p2trgDMbx...
If you complete Reach's campaign solo, you'll unlock:Side Note:
There are a few achievements that can ONLY be unlocked while playing on the required difficulty. These can be completed during your legendary playthrough.