Find a good spot that has two trampolines (I used a couple cars) close together near where OD spawn, and equip the Turret Copter or another deployable weapon. Also, use an epic amp that damages enemies. Now, voice on the trampolines, one after the other. When the OD spawn, fire your Turret Copter, which I think is the best deployable weapon for this achievement because of its range. If your style is rank 3, you'll get credit for killing them even though you're not shooting them directly.