I honestly don't understand people's HP threshold for T13. As WAR I personally believe T13 is one of the lower HP requirements in FCoB along with T10.
I usually tank with 9.5K and it's plenty.
You barely have to tank in this fight and when you do it's super short and can afford to use your strongest cool-downs.

Although this is personal because of my group, this is how I do the fight.
I only ever have to use Defiance about 5 times in the fight honestly.
3 times for Shadows, 2 of which are Unchained.
Once for the last Pain, which is Unchained.
And once for the 3rd Ahk Morn, which is Unchained, unless it's the last one then it's Hallowed Grounded.