This achievement can be obtained naturally by completing the main mission "Colony Gone" at an early stage in your career. After battling through tons of no-organic Geth the final two sections: Minimize Casualties & Destroy the Thorian will be chalk full of Thorian creepers and Asari clones, not to mention a dozen humans if you choose to kill them for the renegade pts.
You'll likely have to repeat some of the five waves of Creepers and Asari clones, but that will only help stack up the body count. All kills are accumulative even if you die.