As reported by GamesRadar, players must reach at least level 46 to unlock the Medusa quest. However, achieving the highest level of 50 will give the character the best chance to defeat Medusa, as all her warriors are level 50 as well.

This mythological creature is the hardest in the game and shouldn't be done until you've gotten strong gear and are at least level 47. In order to find and defeat Medusa, the following quests much be completed: Romancing the Stone Garden.