One of Medusa's specials allows her to create evil replicas/twins of anything that attacks her, including pets, summons, and players. I don't advise bringing greaters to the fight - perhaps not any pets. It gets annoying having to kill greater dragons throughout the battle.
Also, we found it easier to get her up on the second level (via the stairs) and trap here in the corner with EVs and energy fields. When she's trapped upstairs, all the evil twins she creates tend to get stuck underneath her, making it harder for them to reach you.
Another tactic (looks like HOT used this method) is to have part of your group attacking her, while one or two other players keeps the evil twins and such cleaned up.
She's fun. Gl!