This is actually an incredibly easy achievement whose ratio I suspect will change dramatically quite soon. For it, you simply need to go into a quarantine zone and open 5 GRE chests.
What I did was revisit the Parking Lot quarantine zone in the western part of the Old Town map. The objective inside is to kill all zombies, which won't be a problem late in the game (or after you've beaten it). The upside is that you need to clear out this quarantine zone anyway if you want the related achievement. In any case, there are 5 GRE containers inside, all of which have "hard" locks. So just go in, swing away (beware of the exploder zombies shambling about), and open the containers at your leisure after you've cleared away the stinky undead denizens.
It's that easy.
This image is not my property, but it'll show you exactly where to go. Obviously, you want the red rectangle marked "Parking Lot." Bada bing!