This can be gained on DOOM E4M9
- Start the mission and run through the door ahead
- Now go through the next door and don't kill the enemies
- Go through the final door to the RPG and then go back into the previous room
- Again don't kill anything and instead bunch the enemies up and allow the IMPs to fire the fireball at the other enemies as they aim for you. It's actually difficult to prevent them killing each other on this level
If you need more help with this you can follow my video guide below which starts at this achievement: