Amnesia: The Bunker

Thanks, as you mentioned, the key to this is making a save and then re-loading so you don't waste the bullet for this achievement. I'd recommend going into the tunnels and killing the guy with the shotgun first and then going back and saving. Then you can do the achievement. The shotgun guy is the only threat in the tunnels area. The beast doesn't spawn here.

Also random tip for the tunnels, if you're not near the end of your playthrough, I would recommend not picking up the detonator handle, as picking it up will cause a beast spawn to appear in the central bunker save point room.
I went to the tunnels pretty early as I thought it would be great to have the shotgun. I had to load back a couple saves at the main menu to where I hadn't picked up the handle as I didn't want to deal with that for the rest of the game.

I had no idea the beast could spawn in the safe room, the detonator handle is actually the last thing I got, and I didn't see the beast in the safe room when I went in there before detonating the explosive. I'll keep that in mind for my next playthrough though.

For me it was the opposite, shotgun didn't work for me but the achievement popped first try with the revolver. I'll also add that after running out of shells, I went back to where you first drop down into that room, there's a bunch of the vision people lurking about there, and got the achievement when I used the revolver there.

You can definitely get this with the revolver. I made a mistake and used the 1 shell I got for the shotgun on a door and saved too many times to re-load. Took me 2 shots of the revolver before I got it, I think they're just hard to hit as I was convinced I hit my first shot. The key thing here is saving, then using as many bullets as you need to and then reloading the save so as not to waste any bullets.