The key to this achievement is to unload a lot of bullets on a pack of Rakk. There are several things that you can do to make this easier, if you're having any trouble.
1. Complete a mission which requires you to kill Rakk. I unlocked my achievement on the mission in the Arid Badlands which requires you to do so.
2. Play with as many people as you can. The more players in your game, the more enemies that will spawn.
3. Play with lower-level players. While it may not be very entertaining to blast your way through low-level enemies, it will allow you to kill Rakk very quickly.