Step 1: Fast Travel to Three Horns: Divide and buy the weakest weapons available for whatever weapon class you want to rank up
Step 2: Fast Travel to the Caustic Caverns (under Sanctuary) and kill all the Colatile Crystalisks you can find
Step 3: Shoot Blue (the boss of the Safe and Sound quest) BUT DON'T KILL HIM as much as possible. Let Blue kill you when he is close to dead. Lather, rinse, repeat step 3!