Completed it last night with 491 ilvl, and no tier set bonuses.
I adopted a strategy from several different videos mashed together.
First, you have to flask, food, self buff, sac the imp, and get ready for a pre-pot. DO NOT FORGET HEALTHSTONES. Glyphed for speedy enslave demon as well. MAKE SURE YOU ARE USING THE MACROS THAT ARE POSTED. Namely macro your enslave demon to target the pitlord, macro the charge to hit Kanrethad, "oh shit" twilight ward/defense cooldown macro, and macro a self cast felfire breath to hit you (this will cause the pit lord to come to you and breathe on you wherever you are).
I used the demonic gateway threat reduction method. I put one portal RIGHT next to the summoning well, and the other as CLOSE as possible to it in the center of the floor grate.
I then click the summoning well and start from the left side, maybe 20 yds from the summoning portal.
Kanrethad walks out, I /focus, and wait for the countdown. At 1 second, I hit my pre-pot and cast a Rain of Fire on him. He gets tagged and throw up CoE and it's off to the races. I spam the hell out of everything but CB to get up embers (I usually got 3) before the Pit Lord comes. PL pops out, enslave it, hit G (my hotkey for pet attack), then click him to PASSIVE and hit G again. Pit Lord gets to boss, hits him a time or 2, use Fel Fire Breath and hop through the gateway. Now the boss is stuck to the pit lord.
Proceed to blow him up. USUALLY I didn't get a cataclysm before he summons the imps...I'd say maybe 1 out or 5 times I got one.
This is where it gets interesting. One video suggested using the PL charge to halt the DOOM cast that Kanrethad uses on you. To give you extra time and more damage (during the stun). I successfully caught it 4 times in one attempt, but I felt like I was spending too much trying to focus on that and then I ended up getting rattled for the rest of the attempt. My kill attempt though, I did interrupt it the first time (and got off a couple chaos bolts with my synapse springs/Dark Soul for BIG damage).
REMEMBER TO ALWAYS BE JUMPING THROUGH THE GATEWAYS when it comes off cooldown. I did that the entire time and did not have a single chaos bolt fired at me.
DPS hard until he goes to summon the imps.
When he moves to the summoning position, stand RIGHT under the Pit Lords ass. Like he's resting his balls on your head.
Let the imps come out while you continue to dps the boss. Right near the end of the summon cast, hit CTRL-3 to use FFB on Kanrethad and all of the imps. Throw down 2 RoF, run to the gateway, hit Shadowfury, jump through the gate (easiest from furthest gate, which should be closer to the pit lord, that way it throws you REAL close to the giant portal). Once you land, hit the macro for a targeted FFB to nuke the buffs off of you and hit the imps again. Drop another RoF or 2 and the imps should be toast, along with your buffs. Oh and while this is happening, keep ripping off CB's on the boss due to the monster ember generation from those imps.
JUMP THROUGH THE GATEWAY AGAIN (cannot repeat this enough)
Kanrethad runs to the middle for cataclysm. When you see him start to cast, start a chaos bolt. When you're halfway through, hit the charge boss macro to stun him, chaos bolt does monster damage, then you can hit him with another.
After your 2nd CB goes off, start running to the right (if facing the giant portal). Get far enough so that you can click your PL to run behind the big pillar on that side.
Kanrethad will then run to summon the felhunters while your pet is waddling off out of LoS.
Run about 20yds behind Kanrethad (AFTER USING THE GATEWAY), drop 2 RoF's on the portal (spread them a little), hit Unbound Will AND Soulshatter. Soulshatter is VERY IMPORTANT here. If you forget it, you WILL get hit by a chaos bolt DURING felhunters.
With your pet off to the side, your debuffs wiped, and your threat lowered, the boss will run over behind the pillar to attack your Pit Lord.
The 2 RoF's you dropped will pick up the felhunters as they come out. First felhunter hit with immolate, use havoc, cast CB on the 2nd Felhunter, conflag those 2 until dead. CLICK YOUR FOCUS WINDOW AND HAVE THE PIT LORD ATTACK KANRETHAD IN THE CORNER. Then CB/incin the 3rd felhunter one down.
About the time the 3rd one dies, Kanrethad will come sprinting out of the corner to do a cataclysm. IF YOU HAD YOUR PET ATTACK, IT WILL FOLLOW HIM and be in charge range when he starts to cast.
JUMP THROUGH THE GATEWAY, and charge/CB as normal.
Kanrethad then starts to summon the Doomguard. USE THIS OPPORTUNITY TO DISMISS YOUR PIT LORD AND RE-ENSLAVE HIM. When the DG appears, have your pet attack the DG. Let him hit it twice, hit CTRL-3 for a FFB on the DG, then back to the boss. The DG will now be stuck to the boss. Jump through the gateway for good measure.
If CD's are up, dump them into the boss now.
Rinse and repeat. Use a Purification Potion if your DOOM debuff hits 2 minutes and you feel like everything is going well.
Things to remember: ALWAYS jump through your demonic gateway, CONSTANTLY use FFB self target to wipe your buffs. Use singe magic (from imp sac) if the breath is on cooldown. Use unbound will and soulshatter right before felhunters come out. PUT YOUR PET ON PASSIVE WHEN YOU FIRST ENSLAVE HIM.
I ended up somehow glitching the fight, so at the end I had the pitlord up with 2 doomguards on him. Had to spend the next 5 minutes killing both of them. Completed the quest and got the green fire with the pit lord still enslaved, and then just left the instance.