Chapter 1: Blindsided
You have to retrieve Claptrap's eye back. Follow him as he opens the door and leads you outside. Continue looting the boxes and smash the icy skeleton piles along the way. Shoot the bullymongs that will ambush you and continue forth. As you reach the cliff, more bullymongs will appear. Take them out and go down to help Claptrap. Loot the boxes and continue past the wreckage.
The Knuckledragger will appear afterward. Keep shooting at it then run away when it starts to charge. When it is critically wounded, fall back to the wreckage and take out his lesser minions. Once done, continue attacking him. Hitting the glowing blue part on his chest will deal critical damage so aim at it whenever you have the chance.
This is gonna hurt
After defeating the enemies, pick up the loot as well as Claptrap's Eye. Continue up the ramp and wait until the heavy vault opens. Loot the chests inside, including the one in the middle. Talk to Claptrap to complete the mission.