This solution assumes you know how to use the in-game console and that you have progressed past the opening of the game into Sanctuary.
--1. Give yourself ammo (Mini Nukes) and the weapon to launch them (Fat Man):
player.additem 000e6b2e 500
player.additem 000BD56F 1
--2. Give yourself god mode with:
tgm
--3. Now go to an open area and create a new save to be safe and enter:
player.placeatme 00020749 50
Now fire a Nuke at your feet which should wipe everyone out. Repeat until the achievement pops.
If your computer is beefy you could change the 50 in that code to 100 or higher to make it go slightly faster