Did the whole thing as demo (ilvl496 no tier set) found it a lot easier than learning destro but I am a master demonologist so hey ho.
Just save fury/ds for fellhunter and Los the pit lord, use immo aura to get aggro when they spawn, had to take a purification pot just to beat the enrage (was about a minute off). Providing you take a pot it's all about mechanics and how you manage them, excellent excellent fight!
Position gateway on the middle left ring to behind far left pillar
Portal next to gateway
Started with my imp out and popped everything including doomguard
Gosac imp (the dispel is needed) as he summons pit lord
Shadow fury pit lord and enslave
3 abilities on the pit lord:
Stun (interrupts chaos bolt and cata)
Flame breath (removes his dots)
Heath funnel (heal)
Stunned and breath as soon as enslaved
Dps standing in breath but not under his RoF dispelling his his dots
Just about make a 2nd stun on the chaos bolt but might need to portal to gateway and Los the cast
3rd stun when he does cata
Position pit lord right in front of his portal and stand between it and and 60 imps. Breath on the imps and cast GoH/immo aura in meta. Usually used sac pact plus pit loads heal
Portal to gateway and Los the chaos bolt!!
Stun his cata
Breath to remove dots and heal
Position pit lord on far right behind pillar (might need to do this early as he's slow)
Stand behind his portal casting immo aura to aggro felhounds
Nuke the fuck out of the fellhounds ASAP use shadow fury wimps all cd's
*wipe -many times-*
Portal and gateway Los chaos bolt
Attack him with pit lord -you might need to re-enslave-
Stun cata and breath to remove dots
Dps as normal
Re cast portal next to gateway
Position pit lord infront of his portal
Shatter when the doomguard is summoned and attack it with pit lord
Portal to gateway and Los chaos bolt
Potion of purification to dispel ultimate doom after 2nd fell hunter phase
Enslave on the pit lord last only 5mins to I left one fellhound up when Los chaos bolt to eat it left me just enough time to enslave and interrupt cata.
Remove his dots constantly (they can dot a lot of damage plus stun/void zones can screw Los the chaos bolt, bad times) basically keep using the sac'd imps dispell and running close to pit lords breath
You can banish one of the doomguards but I found letting the pit lord tank was easier
LoS chaos bolt can be tricky, I had a few wipes due to be running back before he finished casting it or being in mid gateway flight when he finishes it, I could just about take one with unending resolve and twilight ward followed by some fast healing!
It's probably the most challenging thing I've done. Meant to be a lot easier as destro but I wasn't hitting 20% by the 3rd wave of imps but with new lfr gear it should be very easy if you get the mechanics down which is by far the brutal/unforgiving of any encounter/raid I've done. Ever.