In that tamer setting i'd be a little concerned by your ability to keep the greater dragon alive but if you can keep mana regeneration up then it will be no problem.
I tried different "taming ways" to kill medusa and what you really need to get rid of her is basically a lot of mana/mana regeneration, 95 magery and a greater dragon.
Hide when she retargets, keep healing the pet from distance and it's done.
The safest way i found to do defeat her is with tamer-mistic (take her in a corner, healing stone, stone form, spam bandages+cleansing winds on the pet) but it is very, very, very boring - takes more than 1 hour.
Of course you have to get rid of the pet-copy as soon as possible, a thing that is not that easy with the dumbchimp pet behavior (all stop, all kil - the copy, what happens is that the dragon hits the copy once if ever then immediately retargets on medusa... very annoying).