We're off to the Island of Lesbos in the north east corner of the map. On the southwest part of the island is a quest called Romancing the Stone Garden.
You'll end up in front of the temple and be sent to retrieve some items a spear and a key. The spear is on a mercenary and the key is on the island of Chios in the Huntress Village. If you've finished the Daughters of Artemis questline you can walk right in and grab the key. Once you have the key and spear return to the temple in the Petrified Forrest.
I recomend having your torch out as you wander through the temple before the fight to help spot snakes before they attack you.
Once you get to fight Medusa she switches between shooting a laser at you and exploding the ground you're standing on. The laser is easy to avoid just keep one of the pillars in the arena between you and her while shooting around it. When she teleports after taking damage be prepared to dodge her AoE ground attack. That is really all there is to this fight but it is a long one. Don't get impatient, keep yourself healed and if you need more arrows there are a few racks around the arena.