Curse Magic
This basic attack not only inflicts damage (easy to deal with) but also applies a stack of Poisoned. You can have up to 10 stacks of poison and that will, almost in every situation, kill your units.
She applies this to 3 units at a time. Stacks “only” last 4 seconds but since this is her basic attack, they have almost 100% uptime.
You need to run damage reduction units and use defensive pets if you are dying too often to this poison, lowering your damage a bit but allowing you to stay alive the whole fight.
Petrify
Inflicts damage (pretty easy to tank) and also petrifies the whole team for 3 seconds. 13 second cooldown so she can use this pretty consistently.
This paired with the Hatred passive is what makes this boss super annoying to deal with (and she also has poison damage that stacks).
There is no real way of fully dealing with this since it’s only a 13 cooldown skill. Units like Merlin and Tristan help with their CC. Pets like Esteria also apply fear every 20 seconds.
Other characters that will help make this fight easier are shielders like Erin and damage reduction units like Galahad and Isolde.
Hatred
Increases her ATK when petrifying an enemy. This is pretty dangerous since it’s 100% and she can stack this up.
Mystic Power
Medusa’s damage reduction increases with each hit. 1% per stack, up to 20 stacks. Nothing you can do with this outside of running ignore damage reduction gear / pets.
Knockback Immunity
A passive you don’t have to worry about at all since we barely have any units in the game that can move bosses.