Ok this work perfect..be sure to do a couple of things, i'll try to explain what i did:
1- as stated in the first post, buff up..set the portal down behind the corner and set the demonic gateway as shown
2- let it absorb the first chaos bolt while the bot does it's usual rotation (this help with the medium dps)
3- start dpsing the imps standing between the boss and the imps so the pit lord breath will hit them too and you'll get cleansed
4- after the imps died be sure to be near your teleport because you're gonna need it for the chaos bolt
5- after the teleport refresh it just to be sure
6- before the felhunter phase (2-3 seconds before it actually start casting,you need Bigwigs Bossmod addon(i dont remember if it's bigwigs or littlewigs for that scenario..try both if noone of them work try Deadly Boss Mod)) use the macro that release the pit lord and then mindcontrol him again.
7- after he's again mindcontrolled stop Tyrael with the Hotkey that usually is Alt+x and set manually the pitlord to go where your portal is (THIS IS CRUCIAL)
8- resume the combat routine...ALT+x again target the felhounds and keep them in front of the portal out of los of your pitlord and let the bot kill them
9- after this phase keep doing what you were doing always avoiding chaos bolt standing near the teleportation (when you'r teleportation spell isnt gray youre in the correct spot) because the bot will teleport you asap f there's a chaos bolt but clearly you have to be in range of it
10- when the last add comes let the bot banish him and keep dpsing the boss.
Rinse and repeat until he's dead.
Once in a while when the situation is safe you can use the macro to refresh the duration of the mindcontrol.
Last thing i have to say is
ALWAYS TRY TO STAY IN THE CONE OF THE PITLORD FOR THE DISPELL AND OUT OF THE BOSS RAIN OF FIRE
You have to get used to it so you have to make some practise.
This is how i did it with a 515 ilvl warlock in like 10 attempts
Hope this helps someone