Watch out for the poison that encircles the battlefield. If there's poison all over, it means you didn't burn the windmill earlier in the area. If you die, make sure you go back and burn the windmill to remove the poison from everywhere except the outer edge of the battlefield, as this will make your life a lot easier.
Stay close to Mytha unless she backs up to the edge of the battlefield where the poison is found. She'll attack with three-hit combos using her spear as well as a full-body spin at close range. Both of these attacks can be blocked with only moderate stamina loss. However, there isn't much delay between her attack cycles, so you won't be able to hit her more than once or twice depending on your weapon of choice.
If she backs away, prepare for one of two attacks. Expect either a dashing spear attack that can be blocked or evaded to the left or right, or a magic attack from her head. If you see her head glow, prepare to move quickly to the left or right, then dodge as the attack approaches. The magic projectiles move quickly, so you won't have much time to react.
If you stick close to Mytha, you'll avoid many of these attacks. However, she will back away regularly, and you'll also need to be mindful of the poison surrounding the battlefield since it's easy to stumble into if you're not paying attention. Just be prepared to handle her ranged attacks and stick close to her as much as you can, and this battle isn't all that difficult.
Once Mytha is defeated, head across the poison, through the doorway and up the stairs. Step inside the lift, then through the doorway to the right when it reaches the top to finally enter the Iron Keep.
If you need more information and help, the rest of our Dark Souls 2 walkthrough and complete guide can be found here.
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