You'll get this achievement for completing Mission 8: Eyeless. In this mission the demon Phineas will ask you to retrieve his missing eye. To do this, you'll need to chase a couple of Harpies to their nest, in which the eye is located.
The road to the nest is crawling with enemies, ranging from standard demons to several shielded ones. Be careful when crossing the railroad, as there will be two trains coming at you.
Once in the nest, you'll be ambushed by several harpies and Pathos'. Deal with them any way you see fit, but make sure you don't fall to their death when the glass around you starts to break.
On your way back you'll encounter more standard enemies at the intersection, and three more (including a Tyrant) right before you get back to Phineas.
Once you reach poor old Phineas, a cut scene will play and the achievement will be yours.