FSIRT Against the Wall Achievement
FSIRT Against the Wall20
In Buried, purchase the LSAT off the wall and Pack-a-Punch it.
The LSAT is founded in the starting area. You will need to survive for two or three rounds to amass enough points to buy it. When you have enough, go to the broken staircase. Hug the left side of it and jump. Press to grab onto the ledge then quickly buy the LSAT on the wall. As soon as you do that the ground will break. From here, just go through the normal steps of Pack-a-Punching a weapon. You can keep it inside the wardrobe until you are ready to Pack-a-Punch it to avoid potentially losing it.
This video also contains a guide for Revisionist Historian.