Stack as much +/- % Damage and +/- % Damage to/from Demonic enemies as you can. Immunity to Petrification is HIGHLY recommended due to his triple gaze attack that comes from random locations on the screen. While it is possible to evade these attacks, having immunity will let you focus on dealing damage. Note that you just need to turn your back against the eyes of Medusa and then you wont be petrificated.
He has attacks of all elements at his disposal. His ice attack hits hard, his lightning attack stuns, and his fire attack covers a large area. Occasionally he will spawn a purple circle on the ground which drains your health incrementally, with each tick being transferred to him as 2500 HP.
Work hard to keep yourself and your allies alive, reviving when necessary. Having a large amount of money will be very helpful as it is likely that you and your party members will die multiple times. Find other players to help you as NPC's will die very quickly.