Relevant Talents used:
Angelic Feather
From Darkness, Comes Light
Desperate Prayer
Divine Insight

Credits to caspqwerty for his strategy for this boss. Basically the aim is to kill Sartharion before Shadron makes him immune to all attacks, and the DPS requirement to achieve this is quite immense. However it can be done with a little trinket swapping trick.

First, Shadron is engaged purely to build up 3 shadow orbs. Once 3 shadow orbs are obtained, go through the portal to reset the fight.

Before pulling the boss, equip the Seal of the Seven Signs into trinket slot 1 and put renew on self to trigger the proc. Once the proc is triggered, re-equip trinket slot 1 with whatever trinket is there before. Trinket slot 2 should be Insignia of the Corrupted Mind. And yes, the haste proc from both trinkets do not share a cooldown and stacks. Once all that's done, pull the boss.

Shortly after the first Fire Wall, start kiting the boss towards the entrance, when Tenebron gets in range, send Shadowfiend on it. Start kiting the boss again once the Shadowfiend is defeated. At this point, it's a matter of using instant casts and SW:D to finish the boss. Angelic Feathers are used to facilitate the kiting process.