The Gorgon Warmistress is the third boss you will face and its element is Water/Earth so it is weak against Air and Spirit. You can face it in the Relic Ruins and at the Summoning Stone. If you beat it 10 times, you will get the achievement Gorgon Gladiator.
Gorgon Warmistress EditElement: Water/Earth
Max health: 142
Summon cost: 6 Gems