The earliest you can get this achievement is in in DOOM II's MAP02: Underhalls where the Super Shotgun is first introduced.
To get the super shotgun:
From the start of the level until you flipped a switch to open a barricade to enter the eastern tunnel, you'll find a fork on the left to a UAC door to the computer room. The room will be full of zombie riflemen guarding the super shotgun.
The best chance to earn the achievement:
There are lot of enemy zombies and imps in this level. Even more if you play on Hurt Me Plenty or higher. Try to rope them all into one group by running around them until they accidentally hit and start fighting each other. Your super shotgun blast will be enough to down them all from the right distance because of how widespread the blast is and how little the enemies' health are.
Tips:
This level has a lot of enemy shotgunners, so they can accidentally hurt each other and drop their weapons and ammo upon death for more chances of you earning the achievement.