On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled This Town Ain't Big Enough. This is one of the several side missions that you can unlock during the exploration of the Southern Shelf location.
In Southern Shelf, all side missions are handed out by Hammerlock after you complete the Cleaning up the Berg main mission, with an exception of Handsome Jack Here!, which activates after you find the ECHO Recorder.
This Town Ain't Big Enough
2) First Monglets group
3) Second Monglets group
After talking with Hammerlock, head to one of the two spots marked on the map:  or .
In order to reach , you need to jump down from a bridge shown above.
The second group of enemies  can be reached by going north-west of Hammerlock. There you will have to fight a group of Monglets. After clearing both places, return to Hammerlock to receive the Angry Rifle. Two more missions will become available. Accept them both and head to the western gate.