To get this achievement you must use the reload of a Tediore weapon. When you reload these weapons they are thrown rather then the mag being ejected.
To acquire a Tediore weapon fast travel to any weapons vendor and see if they have one for sale, if not head out to another one. You can also get these weapons via chests or random drops from enemies.
Next head out to where some flying enemies are. I personally found Rakk to be easier to hit as they dive bomb you. Eridium Blight and Arid Badlands are the best farming places. Now damage your Rakk and reload when it gets close :)