Thanatos Tower
Floor 1 - Guardian's Hall
Overview
This fight will require someone to tank/kite the main boss, Rathgricy, while the rest of the group destroys each of the crystals on the left and right sides of the map. Once destroyed the main boss can be damaged.
For this fight you should probably have a tank, healer(s), and many DD's.
There are a few trash packs to clear out as you work your way to the northern part of the map. After you aggro Rathgricy phase 1 of the fight will begin.
Phase 1 - Destroy the Towers
After aggroing Rathgricy the tank should take the boss north away from the party so it doesn't bother the rest of the group while they destroy towers.
Meanwhile a mob will spawn in the center of the map with either a blue or red aura. The main group should run this mob over to the crystal of the opposite color. So if the mob has a blue aura, run it over to a red crystal.
Make sure to pull the mob close enough to the tower so that the aura overlaps it then kill the mob. Be warned, these mobs explode when they die and the aura indicates the explosion radius so be sure to get out of it if you're squishy.
The group should watch out for Temple Overlords and kill them on sight as they can repair crystals. Once all four crystals are damaged Rathgricy is no longer invincible and you can proceed with phase 2.
On Brave difficulty or higher, towers have shields. The group needs to destroy these shields before killing the temple guard otherwise the explosion will deal no damage to it.
Phase 2 - Rathgricy
It's just a DPS race at this point. Kill the boss before its energy reaches 100% or the entire team will wipe.
The boss will leave holy fire fields on the ground that deals damage while you stand in it. Most importantly, her energy raises while people stand in it so be sure to move out as quick as possible to avoid a wipe.