This achievement is self-explanatory and should come through natural game progression. However, if you are nearing the end of the game and still don't have it, or are just confused, try one of these methods:
1. Set up shop next to a weapons store and start wreaking havoc until demons arrive. Use your imps and wipe your notoriety when you see fit as more powerful enemies will begin to arrive. Rinse, and repeat.
2. If you have any available, hop into a survive challenge (or an altar challenge perhaps) and once again only use summons power to kill enemies. Some activities including survive and altars can be restarted again and again for an endless supply of enemies.
With either of these methods, it should take no more than ten minutes. This achievement is even easier when you have your summon power upgraded, as you can bring in up to 4 imps, among other things, to wreak havoc upon the demons.
That's about it. Please vote if you found this helpful and comment if you didn't.