This can be gained on DOOM E1M3
- You should gain this one naturally as you play, but otherwise start the mission, quick save and pickup the shotgun
- Go through and try lining up the enemies as near as possible so you can shoot 2 with a single shot
- Do this 3 times for the achievement, this can be across multiple games and doesn't need to be done in one mission
- If you struggle or die quick load your save and get back to it
If you need more help with this you can follow my video guide below which starts at this achievement: