This is a three part mission. For the sake of those who like a shocking surprise, I am using spoiler tags for the shocking parts.
The first part is meeting with CL4P-TP AKA Claptrap in his little dumpster haven in Sanctuary. He will agree to help you get the vault key from John AKA Handsome Jack. Meet him at the deterence field in Thousand Cuts. That's the big red energy wall that kills you if you attempt to pass through. Doesn't hurt Claptrap, though and he disables it for you. Once through you must begin a long climb up a road to reach your next goal. You are going to have to deal with a massive amount of Hyperion engineers and loaders, some auto turrets and surveyors. Friendly Slab Buzzards will give you some help, but you will be facing a brunt of the Hyperion Fury on your own. The top of the first hill gives you a Badass Constructer to deal with before a door opens, leading to the second hill climb where you must dodge mortar fire. at the door at the top of this hill you will face more engineers and loaders, through it and up some stairs another badass constructor awaits. Once all the enemies are vanquished you will move on to the bunker.
The Bunker is a tower structure with spiral ramps and stairs that lead to a large flat surface. Brick will inform you that you must destroy the auto cannons so his Buzzards will stop getting shot out of the sky. After you take out the first cannon, loaders will spawn and floor lasers will activate. The loaders can be fought or ignored, but will continually be replaced by more. Your focus should be the autocannons. The laser grids damage friend and foe alike on the upper platform and on the outer catwalks. The stairways are free of lasers, but jumping seemed to work fine for me to avoid most of the damage. There are 12 autocannons in total and all must be destroyed before moving on with the mission. Spoiler Alert Number One!
You can now access the elevator down to the control core, Angel and the vault key using Jack's voice mimic and the password given to you now, by Angel herself. When you get down here you meet Angel. Spoiler alert number Two!
To continue forward you must destroy three Eridium injectors. The injectors are shielded, however, and protected by respawning turrets, shock towers on the floor, and waves of spawning loaders. There is plenty of room to keep moving, however, and I recommend a clockwise or counter clockwise sweep around the area taking out any enemy that comes into view. At various intervals, ammo will digitally spawn, though I never really needed it. Roland shows up to help out by disabling the shields on the injectors and fighting alongside you with his healing turret and helpful banter. Lilith also makes an appearence, although she was expressly told not to by her crush, Roland, and they argue about this. She helps disable the shield and fight the good fight like Roland. Tougher enemies appear, but with two extra vault hunters at your back, things get easier, not harder. Once all three injectors are kaput, you go into a cutscene to complete the mission, which you can turn in later. If you do not want the shocking surprise ruined DO NOT REVEAL THE NEXT SPOILER! If you don't care and just want to know what to expect, go ahead, but I recommend you DO NOT SPOIL IT! IT IS SHOCKING! *Insert Dramatic Sting*
SPOILER ALERT NUMBER THREE!