I got this on accident when fighting the Broodmother boss (I was level 13)
Run around the area breaking eggs which count towards "Abstinence Kills" and "Demonomicon", and when you have built up your Overdrive (I had shockwave), you should have several dozens of spiders behind you, turn around and use it.
You should cause a ton of damage with this, killing upwards of 50 easily.
Some other suggestions:
- high strength build, bonus attack power
- high crit chance
As you level and acquire better gear, you should get this without really trying.