Bros to the Close is an achievement in Halo 4 and Halo: The Master Chief Collection. It is obtained by making it through the fourth campaign mission without a single preventable Marine death on Heroic or harder.
You do not have to play through the entire mission and may start at Rally Point Bravo. There are a total of 6 Marines that you must keep alive during the hill defense.
Tip: You will want to stay ahead of the Marines in order to draw the Prometheans fire, you may also want to give the Marines Promethean weapons before inserting Cortana into the console.