Basically you will activate a nearby spawn location and visit the shed in the middle of Cornivora where 7 Eridium crystals are close to a red chest, which will spawn the default amount of about 3 each, even on the first playthrough – no mayhem needed.
Drive over there, collect everything and quit to main menu.
Select play, continue, grab the vehicle and drive to the spot again.
Repeat this for as long as you see need.
via YouTube Borderlands 3 Efficient Eridium Farming FAST PS4Pro HDR