This is one of those achievements that takes some time. How much, you ask? Well, it spans the entire single player game, so quite some time. The Achievement Hunters are on the case, though. In the videos below, you'll see how to gather all of the enemy intel in the first two-thirds of the game. But what about the last act of the game?! They're working on it, and once it's up I'll be updating this post to include it.
In the meantime, get to finding those pieces of intel. The reward for finding them all is 35G on Xbox 360, or a silver trophy on PlayStation 3. PC guys, you only get bragging rights. I guess that's what we get for having mouse look instead of using an analog stick to aim?