Any time you're playing with multiple characters, there's a chance for this achievement. Eventually, one of your party members will die. As long as you have points remaining in your "Life Pool" (which is represented by hearts directly below the timeline at the top of the HUD) you will be able to revive the downed character. Simply move one of your surviving characters to the hex where the dead body is and press A to revive them.
If you're playing a multiplayer game, the person controlling the character being revived will also get this achievement.