OMG hes a pain to get. Its not just Caustic Caverns, its anywhere theres Varkid. Caustic is just easiest. Use the badass varkid in the dump truck at the start. Let him go in his pod and become super badass. then let him do it again and he will be ultimate badass. then you have to kite him over to other enemies, threshers work best, and like a goliath when he kills enough he will evolve into the boss Vermilicous or something like that.