You can get this while using a rune, and there's one that makes the trophy easy. At level 18, you can choose Teeth as the rune for Bone Spear. This decreases damage, but increases rate of fire and spread. Since an enemy can count more than once towards this achievement, you only need to find a medium group of enemies and hold down whatever button you've bound to bone spear. I easily got it on my first try with the Teeth rune.