After you’ve repaired the Claptrap, continue on your journey to the east. After a short distance and a handful of enemies, you’ll reach a third New-U station, a red chest, and two gray chests. From there, there are a few paths you could take, but the best way to go is the stairway to your left. It leads to some more chests and a catwalk that gives you a good vantage point on the enemies below. After you’ve cleared out all of the enemies, make sure your ammo and health is full and your shields are recharged before heading into the large circular arena.
In here, you’ll battle a massive level 15 Roid Rage Psycho, as well as several midgets. The boss has a variety of attacks, but you can effectively avoid them all if you keep a safe distance and circle around. Headshots deal twice the normal damage, but they’re hard to pull off while you’re moving, so don’t get yourself killed while trying to line one up. If you aren’t a master marksman, just keep firing into his chest. Elemental weapons and powers work especially well against him.
If you’re a hunter, soldier, or siren, unleash your combat skill against the midgets right away. If you’re a berserker, save your berserk skill for the boss. You may want to clear out all of the little guys before focusing on the boss, but don’t. Having a few of them around is great for when you need a second wind.
Once everyone, boss included, is dead, hit the switch in the middle to bring up the mine gate key. Before you leave, make sure to pick up everything that was dropped by enemies.
The quickest way to leave is to exit from the pause menu. Once you reload your game, you’ll start at the entrance to the safe house. If you feel dirty leaving that way, a shortcut has been opened in the form of a now-unlocked door. It’s just past the last New-U station that you passed. You won’t be able to miss it, as guys will attack you from there. If you get turned around on your way back, which can happen, just check your map.
Article from Gamersyndrome.com