As I mentioned on my solution for upgrading your stealth weapon these require you to pick up a new weapon for that catagory as opposed to actually upgrading them at a work station like in the previous games.
Note: This is during mission 6.
This one is just after completing the objective to turn off the power when you are given the keycard locations.
You will go through a door and down into a house when you come out of the house there will be 3-4 zombies in an enclosed area. One of them is a spitter and you have to kill him to get a keycard to open the gate and proceed. When you go through the gate on the right there will be a worktable familiar to people who played previous games. It is not interactable but on it is the axe which gets you this achievement.