Eridium is the rare mineral in Borderlands 2 for the PC, PS3, and Xbox 360, used for augments, upgrades, and to purchase some of the most powerful weapons in the game.
If you’re in need of a lot of Eridium, then you’re in luck, because there is a spot in the game that you can go through as many times as necessary to stock up on Eridium.
In Three Horns Valley, you’ll come across a boss named Bad Maw that drops one piece of Iridium when you beat him.
Kill him, get your Eridium from him, then enter the Bloodshot Stronghold and Save/Quit.
When you come back to your game, go back to Three Horns Valley and kill Bad Maw again.
Repeat this process until you have all of the Eridium you need. It might be a slow process, but it’s reliable and worth it in the long run.