This can be gained on DOOM E3M1
- Start the mission and press the pink eye/skin button and kill the enemy when the platform raises
- Use the left skull button at the side of the door to open the door
- Go through the door and follow the passage left to grab the shotgun
- After grabbing the shotgun use the far left side to come back around and out of the lava
- Now go back and take the other passage which was on the right before grabbing the shotgun
- Kill the 3 enemies and continue through the door into the passages
- In the passage kill the demon and then continue through into the next room
- In the next room pick up the 4 armor pieces and 2 blue health potions
- Go through to the next room and kill the 3 enemies
- Before you use the exit door, face the exit door and the wall on the left is a secret door, open it
- Grab everything inside and you can then finish the level through the exit door and gain this achievement
- Shotgun, 4 armor pieces, 2 blue potions and the RPG should be all that's required
If you need more help with this you can follow my video guide below which starts at this achievement: