How did Argus kill Echdnia?

(Argrive) Echidna was a half-woman, half-serpent Drakaina (She-Dragon) who murdered travelers passing through the lands of Argos and Arkadia. She was eventually destroyed by the hundred-eyed giant Argos Panoptes; he waited until Echidna fell asleep, and then killed her.

Echidna's father was Peiras, son of King Argos, and her mother was Styx, the nymph of a deadly Arkadian stream

The Argive Echidna was sometimes identified with Echidna of Arimoi, immortal wife of the giant Typhoeus.