He is the final boss in the Refuge of the Apostate. Tian-Bai is an illusionist and has built something of a labyrinth in the refuge.
He is a caster boss, and as such, primarily attacks with magic and spells. He has one major AoC attack, Fury of the Betrayer, which can do quite a bit of damage. Clothies and non-soldiers would be well advised to hide behind the pillars when he casts this.
When fighting Tian Bai, normal attacks and combo's are sufficient. He attacks with magic so any protection gear or buffs are very useful in this fight.
Every 25% (at 75%, 50% and 25%) he will stop attacking and start casting Fury of the Betrayer. This is an AoE attack that hits in all directions. If you are a soldier class, then the use of cunning deflection can mitigate the majority of it's damage, and you can simply keep on attacking, whilst consuming health and stamina potions. For clothie and dps classes, it will most likely be neccessary to go behind one of the pillars, wait till he casts his AoE, the return to fighting him. This tactic can be used by any class, of course.
Note that as soon as he starts to cast Fury of the Betrayer, he will port the player to his position in the middle of the chamber and put a snare on the player. This designed to make sure the player is hit by the AoE. To get behind the pillars in time you simply have to hop (double-tap) your character behind the pillars.
Tian Bai Boss Fight