Sartharion with three drakes up on normal is arguably the most difficult encounter in the game at the moment. What makes the fight so hard is that not only is it healing intensive, you need massive DPS and must have three tanks.
However, there is hope. Since the warlock buff, you can swap your main tank out for a Voidwalker. A warlock from my server was successfully able to tank Sartharion easily using his voidwalker. The key is to use Void Star Talisman, the trinket from Tempest Keep, increasing the Fire Resistance of your voidwalker by 130. Void Star Talisman - Item - World of WarcraftBy buffing the Voidwalker to the max, he can reach upwards of 70k health, with amazing passive fire resistance and damage mitigation. With the subtraction of a tank, you can add another much-needed DPS and make the fight substantially easier.
Essentially, the Voidwalker stays in place, takes the firewalls, and survives. It is generally easier to heal the Voidwalker than even a well-geared warrior tank. If the Warlock spams health funnel on his pet (with improved health funnel of course), it is virtually like the pet has a mini shield wall up at all times. Please take some time to look at the pictures below.
And u need this here
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And here the Video
[YT]http://www.youtube.com/watch?v=-uR4zYa6cvM[/YT]