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 kill the boss within a certain number of rounds. This requires a main damage dealer as your carry and it should be at as high a level as possible. Victory Conditions:

Have 2 Saints with more than 50% remaining HP. Try to have your main damage dealer at a high level, as this makes it much easier to complete the instance. The 6-round limit requires your main damage dealer to be able to deal significant damage. When the boss starts summoning monsters, make sure to focus it down as fast as possible to prevent more monsters from being summoned.

III. Recommended Level:

It's recommended to use Saints of at least level 45. Marin and the main damage dealer should be at an even higher level. Saints above level 40 can also be equipped with special Cosmo. Remember to fill all available sockets on your Saints as this will make the instance a lot easier.