The fight is a real pain, just completed it myself but only just beat the soft enrage with 496 ilvl gear, it'd be near impossible with dungeon or pvp gear without some sort of nerf. Main thing I can say is enslave the pitlord he summons if your not doing that already, he has several abilities that are mandatory for the fight. Use the pitlords fel flame breath on cooldown to remove the damage buff on the boss and do a good amount of damage, save his charge interrupt for cataclysm. Put a portal behind a pillar and line-of-sight whenever boss casts chaos bolt. Use GoSac w/ imp and self dispel the agony dot he puts on you. Other than that just kill the add waves as they come at you, except when he summons the doomguard, keep that banished throughout the fight.
When he summons felhunters, thats the toughest part of the fight since they dispel your enslavement on the pitlord. Save dps cooldowns for when they are about to spawn and burn them down quick and re-enslave the pitlord. I highly recommend destro for the fight since you can save embers for bursting felhunters and havoc/chaos bolt 2 of them down pretty fast.