the hydra lived in a swamp which had poisonous mist in the air. Hercules had to have a cloth over his mouth because of the poison
Hercules shot a fire arrow which did cut off one of the heads
when Hercules cut one head off he soon realized two would grow back Athena than brought Iolaus to help Hercules fight the hydra. Iolaus would cauterize the stump with fire
When Hercules got to the final immortal head he chopped it off with a sword Athena gave him