Pretty straight forward.
When you go to select a mission at the table in the Inn the option on the right will be to join the Last Stand game mode. Warhammer Vermintide's variation of a Horde mode.
Simply complete a wave and you're done. You don't even need to be there for the full wave. I joined in the middle of wave 4 and we died at some point during wave 5 and I got the achievement.