That's one of the easier items to farm, really. Just start Tundra, run along the left, up the high, ignoring the varkids, take the path up and kill Mrs. Bartlebee or whatever her name is (been a long time). Usually you can do the whole run and save/quit in 3 minutes or less, depending on your gear.
Siren-super fast, phaselock and blast her
Assassin-pretty fast, set up for Crit damage and Kunai. Go stealth and kunai, come out by blasting crit spots. Battle should either be over or pretty close.
Commando, fairly fast. Drop the turret and get in the shield and do work.
Zerker and Mechro, Zerk and Deathrap, ftw.
Don't get it? Save/Quit and restart. Rinse. Repeat.
OR farm the treasure room. You'll likely find Baby Maker and much, much more, likely BETTER items (Baby Maker nerf was pretty harsh) --- XBL GT: Ki11er Six (ones instead of L's and a space between the two words) Treasure Room glitch on Borderlands 2: http://www.youtube.com/watch?v=wuvwzTuUhqc