Left 4 Dead 2

Simply start up the first chapter in The Passing and right near the start in the first set of zombies will be a Fallen Survivor simply kill the Zombie and pick up any items it drops then Hit Start and Call a Vote to Restart Campaign and repeat till you get the achievement should only take 4-5 times.

Also made a video to go with this :)

Even if you don't notice the fallen survivor, the AI will usually make some comment like, "that zombie has something" or words to that effect, alerting you that there is a fallen survivor in the area. Sometimes they will drop one item, sometimes two. If they drop something that you already have, you will not be able to pick it up and get credit, so you will need to use your item first. If they drop two items, just pick up the item you do not have and you will be able to get the second.

Well since everyone has already told you how to do it I'm just gonna give a few tips on recognizing and killing the Fallen Survivors.

Recognizing them is easy, they are wearing the droopy jungle-hats and have grenade or ammo belts across their chests. If you can get up in there faces you can usually see dog-tags and a bottle of pills on their belts.

As for killing them, they are very hard to kill with most guns. Sniper rifles and magnums are strong against them. Melee weapons are also very good options.