As with The Lost Sinner boss encounter, Mytha the Baneful Queen is much easier with a bit of prep work. If you've tried to fight her already, you may have noticed her room is full of poison, which a) heals her and b) makes staying alive quite difficult. But there's a way to get rid of that!
First, head to the central bonfire closest to her arena. Remember the gate that an undead ran by and locked? You can find a way to reach that outer corridor and unlock it for a shortcut. That path also leads to a giant windmill.
You can follow a small ledge to reach the windmill's shaft--and if you use a torch to catch it on fire, there's no more poison. Afterward, return to the boss, and you'll find her much easier to take care of. Need more assistance? Check out the video above.
For more help be sure to check out our complete Dark Souls 2 Boss guide.