You can get this within about 20 min of starting the game. After a few missions, you'll get the objective to get to skag gully and kill Nine toes. Go ahead, follow the quest and do exactly that.
After exiting his lair, just go straight ahead instead of going down the hill to the right, fight some skags in a small valley and and follow the path to the right and up a small hill.
A small flock of or Rakks spawns there. Let them fly away and back to you gain, then kill all of them within 10 seconds to get the achievement. It is easiest with a shotgun. The Siren starts with a legendary shotgun, which is perfect for it.
edit: here is a screenshot of the location. minimap shows were to go: