Each of the five slabs has four upgrades. You get the slab by killing the owner (or potentially, Julia) and then escaping and infusing it so you don't lose it the next day (this means you first need to finish the infusion quest by killing all the Wenjies). Each time you subsequently rekill the same owner (or again, Julia, if you're lucky) you can obtain the slab again, which provides an upgrade, which itself needs to be infused to keep. Five captures of the same slab will provide all upgrades and unlock the achievement.
For me it unlocked the moment I obtained the last upgrade, before finishing the map and infusing it.