some tips that made it easier for me (done it in full pvp gear, mostly malevolent tier, 4% hit):
- destro specc, soul leech talent, sacrificial pact talent, sacrifice talent, unbound will talent, siphon life glyph, enslavement glyph.
- and take purification potions. it allows you to do the fight with lesser gear and more focus on the waves. takes a lot of pressure out of the fight.
- portal behind the pillar next to the gateway ebonlocke comes from for chaosbolt evasion.
- sac imp for dispel.
1) all cds and do as much dmg as possible. i was able to do ~700k dmg here due to missing 50% of my chaosbolts (or so it felt). full pve gear could do much more.
2) don't use gateway. for some it makes it easier but it is just a pain for me. the boss will cast his dots on you no matter what. and it is hard to use the dispel breath without you having aggro and the pitlord standing steady behind ebonlocke. just dispel regulary and he will mostly recasting dots instead of dealing dmg. so soul leech will in most cases be sufficient to stay alive against the boss alone. BUT you have to deal with chaosbolt (he will not cast it while gateway buff is active). it is a personal choice imo what tactic you choose. using portal once in a while is more comfortable to me.
3) imp phase. plant rof in the portal. and regulary recast fire and brimstone immolate for maximum heals due to siphon life. some just use the embers for chaosbolt spam but i die to much with that. perhaps you need 600k hp pve gear to do that so i go the safe route and bomb all imps away with brimstone and pitlord breath as fast as i can.
4) cataclysm. precast chaosbolt before charging to have enough time for a second one and 1-2 conflags.
5) felhunter phase. i lost ~1k gold on this one ... til i got enough gear and read that the hound dispel in a cone and not randomly all around the map as it seemed to me ... use imp dispel or the talent to remove all dots (annoing imp is murder in this phase ...). place the pitlord left of the portal, use soulshatter to reset aggro (ebonlocke will go on the pet and ignore you while dealing with the dogs). position right from the portal. rof on the gateway to insta aggro the dogs. after all hound are there you can let the pitlord attack again. then all cds, havoc, shadowfuryy, chaosbolt, conflag till shadowburn. second one should die pretty fast too. last one "should" die before the next cataclysm as it makes things nasty (not a nonissue with bad pvp gear).
6) doomguard phase. reenslave the pitlord (enough time now). if you want to heal your pet it is now! you will NEVER lose the aggro from a 6 mio insta heal again during the fight. if you just let the pitlord do a bit of dmg (attack for 10 sec + a breath) he should have the aggro for the entire fight and safe a lot of hassle from banish/fear you don't need. so the doom guards are just another dot on your pet. REMEMBER to sick the pitlord on both doom guards again after each reenslavement!
7) as your pitlord should have the aggro from your doom guards the second set of phases should be the same. remember to use the purification potion somewhere around the second doom guard. and don't forget to reenslave (so many deaths because of joy of getting this far then cataclysm happens and pet breaks free and charges you instead of the boss ...)
8) infinite imp phase. if you got here and your pet has the doom guard aggro you have nearly made it. bomb the imps safely and fast until 20%. then all cds, rof the imps, summon doomguard, all shields up and pump those shadowburns. the boss should be rather dead now. remember to resummon your portal! it only lasts 7(?) minutes. and i died 2 times in this last phase because the portal shut down while chaosbolt was cast.
9) if you have to use purification potion and generally have bad dps there will be another deadline in form of a dead pitlord (assuming he tanked the dmg and no fear/banish was involved). i finally won the battle and the pitlord had 10% health remaining.
10) there is a bug (or a feature) that the demons don't die at the end of the battle. so you manually have to kill the doom guards. or fear/banish/ enslave all 3 demons, suck ebonlocke dry, and flee out of the instance (but that is rather unwarlocky).
when you do all the phases savely and often (i had ~100 tries, most of them in blue lvl equip) the last part of frustration is the randomly breaking enslavement due to not beeing hitcapped. especially annoing in the last phase. boss, 2 doom guards, 60 imps on the screen, then fwooosh aggro from everyone and charged stun. even if you survive that and are able to let the pitlord regain aggro ... fwoosh cataclysm and charge is still on cd. hrrrgn.
other then that just wipe some dozen times until you can play the encounter blind. one time it will actually work.