This is a story related achievement and it cant be missed. After the whole ordeal with Joseph tweaking out like a crackhead, you will reach an area where Joseph climbs a ladder and provides cover. The first area you will need to get by some haunted. Joseph usually one shots them in the face but you might need to take one down. Shortly after is a door that is opened by placing your weight on a switch. Once through turn right and destroy the boxes for some ammo, there should be some ammo to the on your path left as well.
Once you start heading down the path a cut scene ensues. A Guardian will free itself. These guys will charge at you but I find their weakness to be grenades. A good tip would be to get the Guardian to try and melee you and dodge it then throw a grenade at it's feet. The blast will drop him to his knees like a $5 hooker. At that point you can unload on him(Believe it or not this wording was not intentional). You probably wont kill kill him the first time he goes down but keep at it.
The 2nd one will appear neat the dead end. Be aware if you go down there with the 1st Guardian alive, he will still come at you and then you have to deal with 2 of them. Another tip is to avoid explosives near the narrow path by the first Guardian's spawn. I have had a piece of a pillar or something break off and land in front. It is a small thing that can be stepped over but the game doesn't recognize that and you get stuck. I have had that happen twice to me.