I. Floors:
The increased difficulty simply comes from higher stats, so there's no need to change your strategy. You still face a single boss, and all you have to do is eliminate it.
Boss Skills:
Heavy Grinder: Deals 100% damage to 1 enemy (The boss' basic attack).
Imprisonment: The boss' main crowd control. It imprisons 1 enemy for 2 rounds. Try to avoid getting your main damage dealer or healer affected by this skill, or you might have a hard time completing the requirements.
Black Hole: The boss' AoE skill. It will grant an extra move if it kills an enemy. To counter this skill, you need to watch all your Saints' HP closely. You'd be at a disadvantage if the boss receives additional moves.
Oblivion: The boss summons Saints and receives skill buffs at certain HP levels. You should try to focus down the boss as soon as possible, since it becomes dangerous when it transforms.
II. Strategy:
This floor requires you to lose less than 3 Saints. So this is essentially a test of your healers and your supports.
Leveling up Eagle Marin makes clearing this floor a lot easier, and don't forget to equip her with the appropriate Cosmo. Our recommendations: Laurel Tree, which has Healing, and Book of Healing, for her Star Cosmo.
To avoid losing too many Saints, you should level up the lower level Saints as well so they can survive the boss' AoE attack.
III. Recommended Level:
It's recommended to use Saints of at least level 50.