Right, back to the arena with you. Where it all started is where it shall end, or something equally prophetic.
Boss - Badassasaurus
Unfortunately, Piston isn't playing fair. Enter the arena and you'll be met by a gigantic, metallic, Tyrannosaurus Rex. Uh oh. Ok, you can fight the beast in the arena. If you plan to do this get your distance from the beast and keep sidestepping, destroying the large missiles that Piston launches as they appear.
If you're having trouble dealing with the monster like this then use the entrance blockade to your advantage. Hop behind this and simply stand there, shooting from the doorway, at the boss. Duck back under the doorway if you find yourself in trouble, and use the nearby vending machines if you need to stock up on anything.
As for phase two, when Piston is on his own on the ground, simply circle around him shooting him with everything you've got, he'll fall over pretty quickly.
And that is the main story of Mr Torgue's Campaign of Carnage! See about spending all of those Torgue bucks, and visit the battle scenes for some more easy quests.