Source of Madness

This achievement unlocks after killing 10000 monsters.
To grind it as fast as possible, you need to find a spot where enemies spawn indefinitely, I've personally found two instances of this.

-In the Cave of Lost Souls, the second zone in act 1, there's a chance you'll find a fleshy section of the cave, to progress, you'll need to destroy three cyst things while enemies fall from the ceiling infinitely, as long as you don't destroy the cysts you can grind those enemies.

-In the Forest of Lies, the third zone in act 1, you may find red small portals wich spawn enemies right below them, as long as you don't kill the portals.

Once you find one of these instances, just aim towards where the enemies spawn, and wait until the achievement unlocks, I'd suggest you use spells that fire continuosly instead of charged spells.