Allow yourself to get shot by standing out of cover. Don't go too far into unsafe locations or out of cover, as you may die. Your fist health bar segment will recover, which is a part of the health recovery. Any health bar segment which is partially filled recovers. After being on the verge of death, wait for this first bar segment to regenerate, so you can take more hits while moving to find health packs. The health packs are white bags with a red cross on them. Some enemies carry them. They're also found on walls as supply kits and at safe jump locations. On mission 2 and onwards, you'll begin to see more of these. You won't have to run around the map as much to find them and may even run into multiple by accident. Once you recover all 4 health bar segments, you'll get the achievement.