On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Splinter Group. You can start this quest in the Sanctuary location after completing the main mission named A Dam Fine Rescue.
2) Moxxi's place
3) Passage to the Mutant hideout
4) Mutant leaders
5) Sewer exit
Head to Roland's hideout and speak with Tannis on the ground floor . She will tell you of a group of fugitives who you should take care of. Firstly head to Moxxi  and pick up the Pizza from the screenshot.
Now go to Bloodshot Stronghold by using Fast Travel. Follow the corridors you will reach the place shown in the above screenshot which leads to the Mutant hideout .
If you're visiting this area after completing A Dam Fine Rescue, there's a chance you will come across a Robot from the above screenshot, which will explode and kill two Bandits. Approach its wreck and take Loader #1340 AI Core to begin Out of Body Experience (details on this mission can be found in the proper chapter).
Keep heading onwards and eliminating enemies until you reach a hatch shown above. After the conversation, the hatch will open and you will have to jump down. Place the Pizza on the table  to begin the fight.
You will have to face four Mutant leaders one after another. After losing 70-80% of their life, they will run away. With the fourth of them (Mick) seriously wounded, they will all appear on the arena. Finish them off to open up the passage through which you can get out of the sewers . Return to Sanctuary to finish the mission by speaking with Tannis .