Left 4 Dead

set up a local game with two controllers. While you are still in the safe house kill the AI bots. Now have your controller with the gamertag shoot the other character once to lower his health a tiny bit. Take the 2nd controller (the one you don't need the achievement for), and shoot your character using a pistol one shot at a time somewhere like his foot until his health gets below 10. If you accidentally kill him it is okay, just revive him and wait till that temp health goes below 10. Now use your gamertag controller to heal the other character who has almost full health. boom, achievement done

Another fairly easy way to do this -- at the end of a level, on easy difficulty, once your team is in the safehouse and before you close the door, just keep getting attacked and having your life knocked down. Your team will not heal you with first aid kits, they'll just keep telling you to get into the safehouse. A few zombies will spawn here and there outside of the safehouse, so it should be pretty easy to ease your health bar down and kill the zombies when necessary. There will be first aid kits in the safehouse, too, in case you need them.

Dead Giveaway - 10G (Original Achievement)

Difficulty: 2/10

The easiest way to do this requires either Two Controllers or you and a friend working together.

What you want to do is make sure the difficulty is set to Normal or higher and shoot your friend or 2nd controller once. Have them confirm that their health is below 100 or simply check yourself if its a 2nd controller. Now pick up the 2nd controller (or have your friend) shoot you non stop until your health is below 10 (use a pistol when it gets close so you don't go too far)

Finally, get out your health kit and use it on your friend and heal them. This will get you your achievement.