If you have a health pack and are low on health, pause game play and choose "Take A Break". While you are idle, you will be healed. When it is done, rejoin the game and the healing will not count against you. Note: This was done under the Easy difficulty setting.
Play on the No Mercy campaign under the Easy difficulty setting. Take a MedKit at the start. You now have two choices: either run off the ledge behind you (hold the Analog-stick Up and you will land on something above the dumpster), or go through the apartment. If you run off, you will be at 64 health (do not jump off or you will be at 2 health). Proceed through the campaign as usual after that. When your health is in the low 20s, your teammates will try to heal you. Take out your MedKit and they will stop. You should be incapacitated somewhere in the subway or sewer level. Afterward, grab every pack of pain pills find. If there are multiples of them, grab one, use it, then grab the other. If your teammates have pain pills and they take them out to give to you, do not take out your MedKit or they will put them away and try to heal you for about twenty minutes. When your health is in the low 70s, your teammates will try to heal you. When you are in the finale, there most likely will not be any pain pills. However, search everywhere on the roof and inside. Break open one of the doors below the door that leads to the Jesus Room. Do not try to open the door to the Jesus Room because you will die trying. Then, contact the helicopter and run for the room you selected and stay there, MedKit out. Your teammates should follow you inside and will keep you covered. Don't worry about the Tank; your teammates will go out to fight it. When the helicopter arrives, you will be at 1 health. Moe to the stairs that lead to the helipad; not the ramp or you will die. You should be incapacitated when you reach the top. Keep the Infected off you and run for the helicopter. Do not worry about getting hit; if you do not stop, you will make it. Since you did not use your MedKit, did not allow your teammates to heal you, and relied only on pain pills (which do not count as healing), you will unlock the "Unbreakable" achievement.