This is for completing the last main mission of the Mr. Torgue's Campaign of Carnage DLC. The end boss is a two stage battle that can be a bit of a pain if your going solo. Heres a few tips.
*** Spoiler - click to reveal ***
Badassasaurus: There are a total of 5 red barrels on the Badassasaurus (two on either side in the front, one behind the head, and another two on either side of the back.) that are the weak spot. Taking out one of these barrels disables the attack its closest to (taking out one of the two barrels on the front/back will NOT disable the whole attack JUST the right/left side of that attack. I.E. destroying the left front barrel will stop a missile from coming out of the left hatch but not the right hatch). The front two will disable the cruise missile attack, the one behind the head will disable the head attacks, and the two in the back will stop him from spinning around and shooting flames. A good idea is to keep on one barrel until its gone and the best ones to take out first are the front two. Those will disable the homing cruise missile attack that can be very difficult to avoid by running but can be taken out early if shot in mid-air (those same missiles can also, with the right timing, be destroyed right as they are coming out which will deal some heavy damage. Check the comments for a more detailed strat to nailing them. [Thanks to Hirsute Dave for the tip!]) The trouble is, if you miss, just one missile can take you down to nearly nothing from full health. (As a sidenote, those missiles can be shot down while in fight for your life and give you a second wind) He does stop for a good amount of time when he is preparing to shoot them so its easy to run up and blast those barrels. If you stay on the barrels and do your best to avoid the near constant hail of attacks, he wont take a whole lot of time.
*** Spoiler - click to reveal ***
Piston: After the Badassasaurus is down, Piston pops out and is very angry. Compared to the previous fight though, he is a push over. He only has a few attacks that mostly include ranged slow moving projectiles and will sometimes charge at you for a goliath like melee attack cycle. The shots come from his hands that will shoot one after the other slowly and sometimes will end with him charging both hands for one big shot. When he starts charging at you, if you keep backing up while shooting and turning around to run when he gets a little too close, you can deal some decent damage as he will continue to charge until he gets close enough to attack but get too far away and he can leap directly to you. His head is the weak spot but once you down the shield, he doesnt have much health to get through. A fire weapon will take him down very quickly. Honestly, he didnt stay alive long enough for me to see him do a whole lot...
After the battle, the vault will open and tons of loot shoots out. When you get all that yummy loot picked up, return to Moxxi and turn in for the achievement.