Mytha, the Baneful Queen

The arena of Mytha is normally filled with poison gas. Fighting the boss in this environment is suicidal because poison can quickly chip down your HP and the boss would punish you while you attempt to heal. You should set fire to the windmill that is pumping poison gas into the boss' arena to remove the environmental hazard.

Speedrun through Harvest Valley and Earthen Peak NG+7, Dark Souls 2 SotFS

Earthen Peak

Start from the Lower Earthen Peak bonfire. Walk up an inclined plane to lure an Undead Steelworker to you and quickly dash down, waiting for the enemy to approach you. In some cases, the nearby Manikin would sprint to you. Defeat the Manikin as quickly as possible, then take care of the Undead Steelworker. Locate a narrow bridge at the further end of which you can find a Grave Warden. Use a bow and arrow to lure the Grave Warden to your location where you can easily defeat the enemy. Do not engage the enemy while standing on the bridge. A Manikin would shoot poison arrows at you while a Desert Sorceress would spam volleys of fireballs at you. Defeat the Grave Warden, then dash across the bridge as quickly as possible. To your left is a ladder, approaching which would trigger Manikin x 2 to drop down from the level above. Be ready to fight the Manikin x 2 and dispatch of them as quickly as possible.

Climb the ladder to find a Grave Warden guarding a windmill. Defeat the Grave Warden, use a Flame Butterfly to light your Torch, and set fire to the windmill. The fire would stop the windmill from pumping poison gas into the arena of Mytha, thus making the boss fight easier because you do not have to contend with the poison gas. The Central Earthen Peak bonfire is found behind the fog wall. Light the bonfire if you want, but doing so is not necessary. Sprint up an inclined plane, then make a sharp U-turn and head for the ladder. Climb as quickly as possible to the top of the ladder, ignoring the Manikins following you. Turn left and head toward an archway, at the end of which is a flight of stairs. Ascend the staircase to find a bunch of large poison jars. Carefully navigate through the room; do not roll through any jar. Stop before the end of the room. Look for a Suspicious Shadow hanging from the ceiling. Do not approach the enemy because the Desert Sorceress to your right would spam pyromancy at you. Instead, lock onto the Suspicious Shadow and spam Crystal Soul Spear at the enemy. Dash as quickly as possible up the flight of stairs ahead and find your way to the Upper Earthen Peak bonfire. Rest at the bonfire and attune any spells you need for the upcoming boss fight. In particular, you might want to attune Great Chaos Fireball.

Head down to defeat all enemies outside the arena of Mytha. Summon Jester Thomas to assist during the boss fight.