On this page of the game guide to Borderlands 2 you will find detailed walkthrough descriptions of the side missions titled Rocko's Modern Strife and Demon Hunter. You can start these quests in the Sanctuary location after advancing far enough in the main storyline.
Brick  will ask you to help out Rocko in Thousand Cuts. Use Fast Travel and head to the meeting point . Speak with him to end the mission.
1) Bounty Board
2) Old mine
Accept the mission from the Board  and use Fast Travel to move to Lynchwood. There, head to the old mine  using the elevator. below you will be greeted by Dukino's Mom and a couple Bandits. Keep a safe distance so that Dukino's Mom focuses on the Bandits. They however won't keep appearing infinitely, so you have to act fast.
The weak point of the Skag is its head and open mouth when it attacks. Your bullets might just bounce off the rest of its body. Dukino's Mom will attack with energy projectiles which can be easily avoided and a beam which you have to jump over. When the animal will jump into the air, it will create a shock wave which you have to move away from. Corrosive weapons will be handy in this fight, dealing damage regardless of which part of the body you hit.
After defeating the overgrown Skag, Dukino will head out of his hideout. Speak to him to finish the mission.