First of all you want to equip a weapon from the manufacturer Tediore.
The fun fact about Tediore weapons is that, instead of reloading, your character throws it away like a grenade and spawns a new one in his/her hand. The thrown weapon explodes on impact or after a short time if no enemy was hit.
When you have a Tediore equipped you want to find some Rakks. You can usually find quite a few in the very first region of the game. Just fire a shot to the ground so you can reload, look up in the air and wait until a Rakk swoops down on you. Now press
to reload while aiming at a Rakk and it should be a one hit kill.
Have fun with your Trophy.