Borderlands 2 Tips

Eridium Farming
If you're not adverse to saving and reloading over and over again to make enemies respawn in the game, one way to continually farm Eridium is to repeatedly kill the "Bad Maw" unique mob located just outside the Bloodshot Stronghold, north of the quick travel terminal in Three Horns Valley.

Defeat Bad Maw, head into the Bloodshot Stronghold itself afterward, then save and quit the current game. Simply reload the file, then once back into the game, leave the Bloodshot Stronghold the way you entered and defeat Bad Maw outside again.

Re-enter the Stronghold, save and quit, and repeat ad nauseam!