Medusa is a monster and is the queen of the Chaos empire. In Campaign mode of Stick War 2, she first appears in No man's land and then at her castle where she is killed by soldiers from the Order empire.
She can turn enemy units (except giants) to stone. She doesn't need to research this ability but it has a long cooldown. She can also create pools of poison (this ability requires research).
Her weakness is range and swarms, the fact that some units can't be turned to stone and you can't petrify 2 men without waiting 27 secs, and that ability has a long cooldown. Use enslaved giants or a 3 man squad of Speartons and Shadowrath to destroy her.