Hydra are killed by preventing the regrowth of their heads after they are severed, and then destroying the remaining immortal head (if one exists).
According to the provided reference, Hercules killed the Hydra with assistance from Iolaus. Here's how:
- Each time Hercules bashed one of the hydra's heads off, Iolaus held a torch to the headless tendons of the neck.
- The flames prevented the growth of replacement heads.
- Hercules removed and destroyed the eight mortal heads.
- Hercules chopped off the ninth, immortal head.
Therefore, the key to killing a hydra is to cauterize the wounds where the heads are severed to prevent regrowth, and then dealing with any remaining immortal head through destruction.
