Surprisingly, clearing this stage is easier than defeating Dark Possessed Zio.

Focus on attacking the lamps to gain souls, but also attack Harsetti by aoe skill. The light elemental lamp heals, while the dark one grants critical hit resistance buff. It's up to you which one you want to attack first.

After Harsetti uses her second skill which activated at the start of battle, your biggest max health hero will become a lamp. To return him to normal, you need to use soulburn skill in his turn which uses 20 souls. That's why you need to attack other lamps to gain souls, cause everytime their health reaches zero, they will become incapacitated and give you 10 souls. Though Harsetti will decrease your souls, it's still enough for you to use soulburn.

For more damage, you can bring hero that counter by aoe attack like Abyssal Yufine, or trigger aoe attack often like Solitaria of The Snow.

When a hero becomes a lamp, he can steal one buff and grants it to all allies by basic skill, and heal all allies by second skill. Although that seems good, better to not let him becomes a lamp for a long time. Cause although his second skill is good, it also increases Harsetti's special buff's number by 1, which when it reaches 5, say goodbye to all lamps, your included. That's why you need to return him to normal as fast as possible. At least, before Harsetti uses her third skill which can only be activated when her special buff's count reaches 5.

Harsetti is immune to debuffs, so don't rely on it.

Harsetti will attack by her basic skill which will be followed by an aoe attack. So, bringing Elena, Last Rider Krau, Edward Elric, Abyssal Yufine, Seaside Bellona, Lone Crescent Bellona, Sol, etc which take advantage of that is recommended.

Even though you don't use second skill of the lamp, Harsetti will grant herself her special buff and increase its count bit by bit. That's why you need to return the hero whom became a lamp as fast as possible, and damage Harsetti by an aoe attack while focusing your damage on other lamps.

Like Dark Possessed Zio, you don't have much time to defeat her after she uses her third skill. Cause by her second third skill, all of your heroes will be wiped out. So, you need to defeat her before that, while she increases the count by 1 per turn,

By unleashing her third skill, Harsetti deals big damage and additional damage, which will nearly defeat all of your heroes. So, for more survivability, better to bring heroes that can revive themselves, or grants immortality buff, or at least one hero that can do that to all allies.

Time to try!