Mytha, the Baneful Queen

Since you've set fire to the windmill blades found outside the central bonfire, the boss area is now free of poison. If you have summoned Jester, you'll immediately notice that he is a potent sorcerer. His fire attacks can deal heavy damage against Mytha, as long as you tank her. You can also go on an offensive if you have a fire-enchanted weapon or can use pyromancy spells as well.

A shield with 100% damage reduction and high stability can help in tanking Mytha and letting your phantom ally do the job. If you're ranged attacker, Mytha has a lot of openings after her attacks and you have to use those openings to either attack or heal. Beware when she throws her decapitated head; it will release an explosive energy shortly. Don't stay anywhere near it after that as her body will lunge towards it to retrieve it. She can also cast magic attacks if you're always keeping your distance. Staying close to her while circling around her will prompt her to continue using her melee attacks.

Avoid using slow weapons unless you're timing is impeccable. Mytha's spear can close the distance quickly and can even hit you even when you're rolling. Mytha's attack patterns are fixed and can be easily anticipated. The main challenge however is how quickly and perfectly you'll react.

Mythusa