Bro Hammer To obtain this achievement you must finish any mission with a Co-op partner on Legendary.
I recommend doing the first mission Pillar of Autumn, the key to finishing this one is using Plasma pistol and magnum combo, hit the Elites shield and headshot them with the magnum.
Also make use of ur grenades which are great for taking out groups and instantly killing elites by sticking them or landing next to them (plasma grenade not Frag).
Finishing a mission on legendary with a Co-op partner will also unlock the following: