For a while I wondered what this achievement was, having used many flares but not having it pop. It actually means, save yourself with a flare when you are in a "low health state" - so when the screen is black and white essentially and you are under a third of health. When that happens, bust out a flare with the and net yourself this achievement.
There is a good chance that this achievement may rely on the fact that you just need to hold the flare, as many have reported that it has unlocked with full health. So upon pulling the flare out with the , hold the button.