Dead Men Walking is a new game mode in Multiplayer that plays similar to Halo Reach's Firefight Mode. As Max and Passos, you and your partner will have to fight endless waves of enemies, while using teamwork and skill to stay alive. Each kill grants you points to your new "Grit" meter, which allows you to open new areas of the map via locked doors, and new weapons and pill bottles via locked containers.
Each container or door will have a icon above it with a different color - bronze, silver or gold. These colors match that of your Grit meter, meaning if you have Silver-Grit, you can open a locked, Silver-Door. For the achievement, you must open 15 of these locked doors/container. Take note that only containers you opened will count towards the 15 total count. Any containers your partner opens will count towards their own achievement.
While doors will remain opened after unlocking them, containers will eventually close and you can reopen them on later rounds. Make sure you and your partner have a clear line of communication in order to stay alive as long as possible.