Here's another one of those big time investment achievements. In Modern Warfare 3's survival gametype, you need to buy all of the items in the armory. All of them. For those not in the know, this will take a while. Especially since you have to level up to get the more advanced and awesome stuff in the armory.
Check out this video by YouTuber alextait1000. In it are some great tips to get this achievement easier. Complete the task, and you'll net yourself 20G on Xbox 360, or a bronze trophy on PlayStation 3.