Have an enemy FROZEN, CORRODED, IGNITED, and ELECTROCUTED at the same time
Borderlands The Pre-Sequel ELEMENTALIST – Achievement / Trophy Guide
Once you get down to Claptraps Skilltree “Fragmanted Fragtrap” you can unlock the Skill “Rainbow Coolant”. It does damage with every element once your stack is empty. You will need a strong enemy for this and is best done on ELESEER: You will find the “Elder Opha” in the center of the map. Drain your stacks by stomping and stand close to her to get the achievement/trophy.