From Final Fantasy Wiki
Echidna is a boss from Final Fantasy III. It is located in the in the final area of the game, the World of Darkness, at the southwest portal. Echidna is one of the four guardians of the Dark Crystals the Cloud of Darkness orders to kill the Warriors of Light. It guards the Water Crystal of the Dark World.
Strategy
Have the Sages cast Haste on the Ninjas first round, then acting accordingly: If she petrifies someone, have one Sage use a Soft while the other one casts Bahamut; if she uses Quake, have them both use Cure4 on the party; if she uses Drain, have one Sage heal and the other one use Bahamut or another strong spell.
DS Strategy
Echnida's main strategy is getting rid of you quickly. It knows Tornado which reduces your HP to a single digit and Death which, obviously, knocks one character out. The jobs for the fight are Devout, Ninja, Knight, and either a Dragoon, Sage, or another Devout. This battle is survivable easily despite the effects caused by Tornado and Death, since Death can be negated using Ribbon or Aegis Shields and Tornado has a low chance of hitting, making Quake and her physical attacks the only thing worth worrying about.
Trivia