On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Defend Slab Tower. You can begin this quest in the Thousand Cuts location after the completion of the side mission named Rocko's Modern Strife.
2) Slab Beacons
Speak with Rock  and head to the container by which you will find three Slab Beacons . Pick them up and place in the designated spots. Each Beacon is a line of defence, by which you will receive Bandit support.
After placing the last Beacon, quickly head onto the roof of the building where the turret is mounted . On the field before you, increasingly powerful waves of Robots will appear. You can't let them reach the container .
As mentioned above, you will be aided by Bandits, including a Goliath. If his helmet gets destroyed, he will become furious and level up each time he kills an enemy. Let him continue his killing spree, as he'll be a very powerful ally. Just remember to stay out of his way.
If the turret gets destroyed or one of the defence lines broken, continue destroying the Robots on foot. With the end of the assault, return to the  and press Use (by default E) on it. That way you will end the mission and collect your prize.