For TEST RE players, this is an 100% walkthrough guide of the game, which will help you to unlock the hidden achievement as well, let’s check it out.
2. Go to 2nd floor and click on the doorknob to open the zinc on 1st floor
and get the key
3. Walk up the stairs for a bit and wait for the column to explode
4. run back to the room where the key is located and wait for the grim reaper walk past the room
5. when the reaper has been seen, run through the stairs to go 2nd floor and unlock the door to get the gun
6. The game END, and all 3 achievement will be awarded (includes hidden achievement)