World War Z

This is what I did....put on Hard mode, not insane. Offline, with AI. I used the Decent level. At the beginning of the level, shot AI player until they fell, and after they are helped up, shot them twice more for good measure, then healed them. I was able to down, heal, and kill more than one AI player per checkpoint reload. Exit back to main menu and repeat 30 times, or however many reloads it takes to reach 30.

This will eventually come with playing through the game solo with bots, as healing the bot controlled characters counts towards achievement progress. Keep an eye on the bots health and heal them when they get really low. You can heal human controlled characters too, but unless you both have headsets or are otherwise communicating they will sometimes run away while you are trying to heal them, lol.

To apply a health pack, stand close to the character you want to heal, and wait for the prompt to come up to heal that person. It will say hold to heal (other character name). If you don't see the prompt, it can be easy to accidentally heal yourself instead, so watch out for that. Don't want to waste those precious medkits. Unless your character needs healing of course.

To apply the stim pistol to a teammate and not yourself, hold in instead of quickly pressing and releasing it. Look at the teammate you want to apply it to, and they will highlight in white. While they are highlighted in white quickly release the and the stim will be applied to that character.

To get the achievement quicker, I recommend playing with friends, and letting each other know when your health has dropped below 20% so you can trade heals and make progress toward the achievement faster. If you wanted to unlock it really fast I suppose you could play with a friend on a high difficulty with friendly fire damage, and just take turns shooting each others health down below 20%, then heal each other back up.