I did the green fire quest when I had a 483 ilvl. It wasn't easy by any means, but I feel ashamed when I see warlocks running around in 510+ gear saying how it's too hard for them (they generally accuse me of lying when I tell them when I did it). I'm here to tell you that you can do that fight in blues and greens as long as you play well. Finishing that fight is much more about managing your agro and paying attention to things than doing super high DPS. Yes, I used a potion, but I still only needed about 3 extra minutes out of that 7.
I very much believe Destro is the way to go. Take Soul Leech, Shadowfury, SacPact, Unbound Will, GoSac, and KJC. For Glyphs I used Healthstone, Siphon Life, Soul Consumption, Enslave, Gateway, and... I forget the last one.
Sac your Imp. Flask, food, pre-pot, and let loose on him at the start until you get to imps, but save your CDs. Once you're enslaving the Pit Lord, make sure you start using your gate exactly on cooldown so he isn't targeting you. Use the Pit Lord's Fel Flame Breath on yourself often to avoid his Seed of Corruption from going off and to keep Agony from getting out of hand. Make sure your UI is set up so you can clearly see your debuffs.
When the imps come out, have the Pit Lord go to them and breath on them. Drop RoF on them, hit Fire and Brimstone, and go to town. Shadow Fury them to stun once all 60 have spawned. Soul Leech here will keep you from taking too much damage, but it doesn't matter how much you take. RoF and Immolate on 60 targets means infinite embers for Ember Tap.
When the boss does his... Cataclysm? Giant green channel in the middle, you have to interrupt it with the Pit Lord's charge. After the imps are down, and you've interrupted it, make your way toward HIS gate. Once he starts running toward it to spawn the fel puppies, hit Soul Shatter, run behind it, and drop Rain of Fire on the center of the gate where they're going to spawn. Since RoF only lasts 8 seconds and it's a 10 second channel to summon, you'll need to re-cast RoF part-way through his channel. The goal here is that the instant the puppies spawn, they turn toward you. If they so much as look anywhere sort of near the Pit Lord (who will be on Kanrethad), they will eat your Enslave and you will die.
At this point you're on your own. Watch your debuffs carefully, and use Singe Magic/Unbound Will to keep yourself clear of his seed. Hit Sac Pact, pop Dark Soul. Havoc the first one that comes out, then Chaos Bolt the second. You'll probably want to conflag after that Chaos bolt, but your goal is that your second Chaos Bolt kills that puppy, giving you the 20% heal from Glyph of Soul Consumption. Try to kill the puppy you Havoc'd with a Chaos Bolt as well, and just do what you can to get the third down quickly. Once you do this, congrats. You've made it through the most difficult part of the fight (seriously).
Next wave is the Doomguard. Before that happens, you're going to want to break and re-enslave your Pit Lord quickly. The fight is timed so that the 5-minute mark from the point of your original enslave is right at the second wave of puppies., which is really poopy. Once you've re-enslaved, have the Pit Lord get a couple quick shots in on Kanrethad (you're still using your gate, of course), and then have him go hit and breath on the Doomguard when it spawns so he will simply tank it the rest of the fight. When you get to the Doomguard, you're free to worry about nothing but pew pewing the boss until the next imps come out. If you make it here, you've seen the whole fight. Sometime near when he summons the imps is when you'll need to use your Potion of Purification to get that curse off.
From this point, you just repeat everything. When you notice Kanrethad is getting close to 20% (probably when you're getting to the second Doomguard with your ilvl), you'll want to pop your Doomguard on him. Pay attention to the enslave timer, because you MAY need to re-enslave before the second Doomguard if you're going to be cutting it close. After the second Doomguard, he only spawns imps the rest of the time (which means it's really easy and you only have to race the curse), but you don't want to have the Pit Lord breaking mid-imps.
Once you can do that, you'll understand why it can be done in blues and greens.
Oh, as for general soloing, I always go Demo.Turn Threatening Presence on the Felguard, DoT everything with Corruption so Glyph of Siphon Life is healing me, gather it all up, pop Felstorm and Hand of Gul'Dan, go Meta, hit Immo Aura, and focus down mobs individually with ToC/SF to get Glyph of Soul Consumption proc.