Fun, Fun, Fun instance with a variety of fight mechanics at play.
Starting off, we've Rom'ogg Bonecrusher whose primary mechanic is to suck everyone towards him, lock them up in Summoned Chains and then execute an instant kill attack called Skullcracker to anyone who stays in close. The key here is to target and break these chains ASAP and then run away. As long as you can manage this, the fight basically cycles through you DPS'ing the boss and executing the break phase to escape that attack. Other than that, he will periodically execute an AoE attack called Quake which, on heroic, also summons multiple non-elite earth elementals for the tank to pick up and everyone to AoE down.
It's worth noting though that Skullcracker can also hit these mobs so it's just as viable to leave them alone and then have someone execute a root on them during the Chains phase to make Rom'ogg kill them for you.
Next up is Corla, Herald of Twilight and you'll find her standing amongst three Twilight Zealot's. The primary gimmick on this fight is that these Zealot's will start getting hit by beams once you pull the boss and, if left to their own devices, will eventually stack a buff to 100 and turn into one of the Evolved Twilight Zealots you've fought earlier in the instance as trash. You really don't want this to happen so what you need to do is have three people, at least, alternate in and out of these beams to prevent them from landing on the Zealots long enough to change. But, and here's the gotcha, these beams work on you too so you don't want to stack to 100 either. Instead you want to let your stacks roll to ~80+, get out of the beam, stay out until your stacks fall off, and then cycle back in. You don't want to let them get much higher than that because the beam effect stacks faster as the stacks increase and, above 90, you're going to be playing with latency fire.
Corla will try to complicate this for you by casting Dark Command and making her target run away in fear: interrupt this. Especially once 4.0.6 hits and you won't have the potential luxury of a constant Tremor Totem to otherwise negate it as that Shaman ability will now be on a 1 minute cooldown with only 6 seconds of use while it's active. If someone does get feared, one of the two people not executing beams should be quick to step in for the person who's running or this should be dispelled quickly. It is possible to have two melee execute as the blockers on this phase as long as the tank positions Corla slightly in front of and inbetween two of the Zealots but, should you find yourself with three melee somehow, the Healer is going to have to step up or one will have to sit out to manage the stacks on the most distant Zealot.
All that said, getting one Evolved Twilight Zealot is survivable with a good tank and healer as long as you make sure to interrupt it's Shadow Step cast which is likely instant death for anyone else in the room. More than that though, and you're going to be boned in short order. Also, if things are going tits up and you're already stacking the debuff, just let it run to full: you'll get released when everyone else is dead and, if you can res, will save everyone the nasty run back to this place. Just be sure to remember to pull back before res'ing... and recall your totems... not that I've ever forgotten to do this... *cough*
Anyways, next up is Karsh Steelbender where your success is entirely determinable by how quickly your Tank can pick up the debuff mechanic. I'll be perfectly blunt: if, as the Tank, you just aren't getting it then loop around the room and move on after a few attempts if you're ready to throw your mouse through the monitor
Ok, Mr Tank, here's your job for this fight. In the middle of this chamber is a large falling pillar of molten lava. You will need to kite Karsh into this pillar and out of it as quickly as possible because doing so is both how you make him vulnerable (in his default state, his armour reduces damage by 99%), and how he damages the party. How? Well, for every two seconds the boss spends in the lava pillar he gains one stack of Superheated Quicksilver Armor but each time he gains a stack he'll pulse a fire wave through the room dealing 1000 damage to the party per stack. So, to make life easy for your healer you want to have him in and out of there as quick as possible - preferably with him gaining only one to two stacks - so that you don't end up with massive bursts of AoE quickly burning down the party. I've seen a few different ways to do this, and you can certainly finesse it if you get used to clipping him in and out of the pillar, but the easiest one is to just turn and run through the lava pillar full bore, not stopping until he comes out the other side and is no longer pulsing.
These stacks stay on the boss for 17 seconds (the tooltip says 12, but this was hotfixed without a client update until 4.0.6), and your goal is take advantage of this spare time to let you have breaks between building the stacks as high as possible without overwhelming your healer by constantly AoE'ing the party. While, with gear and practice, you will find it quite easy to just build stacks slowly to 15 you may need to let the stacks fall off earlier with lower gear levels. Here's where the second mechanic on this fight will come into play.
First, everyone should note the grilled ring around the centre circle; if the stacks fall of this will erupt with lava and anyone standing there will die so it's a good idea to make sure you're always on solid ground. Secondly, as that lava eruption ends three fire elementals will spawn and charge the party. Here's where Tanks tend to make two mistakes:
1: They instinctively pick up the adds; that's their job, right? Wrong. These are non-elites and can be safely tanked by DPS so have them pick them up and kite them to the side to kill. I say this because, when these adds die, they'll spawn persistent lava pools that both impact your ability to move around that middle ring and, more importantly, will also apply the quicksilver buff to the boss if he's in them. You don't want this because the second mistake is to...
2: Panic. "I lost the stacks.... we need stacks... must go get stacks!!!!"
No. The entire purpose of applying the stacks is to let the DPS actually be able to damage the boss but the DPS are currently busy killing adds so having any stacks on the boss right now is pointless. Just hold the boss and wait, let the DPS do their thing and come back to Karsh when the adds are dealt with and then start the process all over again. Doing this does two things: keeps you from adding AoE damage to any melee damage the DPS is taking from the mobs and, secondly, prevents you from having to worry about accidentally losing the stacks again and throwing more adds into the mix. Keep it Simple folks.
All that said, as long as you can manage the stacks this is a pure burn fight.
Our second last encounter is with Beauty the core hound and her pups. The difficulty of this encounter on heroic is directly proportional to how much "Beast" enabled CC you have in the party (and this will get less restrictive somewhat in 4.0.6 when one of Beauty's three current pups will be adopted by: "a wonderful, if not insane family"), and the availability of Tremor totem to mitigate her fear.
With three Beast enabled CC's - assign one to each pup, keep them locked down the entire fight - this fight is a straight up tank and spank with the caveat that, without a Tremor Totem (again, this benefit goes away somewhat with the ability change in 4.0.6), you want to keep towards the entrance of this chamber to avoid people running into Beauty's fourth pup in the back of the room: Runty. Runty's nothing special, but with three dogs already in the room you really don't want to add a fourth. Otherwise, any pups you can't CC will need to be off-tanked and killed before you can get around to killing Beauty because they drop lava pools all over the place that add to the damage being done by the boss herself.
This damage will come from three primary abilities, Beauty will charge and knockback the farthest target from her (do be sure to keep positioned such that you're not going to be tossed at the aforementioned Runty), and will cast Magma Spit and Flame Shock with the first being a fire DoT that will explode upon people around the target when dispelled - move away - and the latter being a proximity knockback primarily hitting those in melee range of Beauty.
So, to summarize, control pups, don't add Runty to the mix, kill Beauty, collect loot.
Coming to the end of the instance we arrive at Ascendant Lord Obsidius and his three Shadow of Obsidius. Here's where success falls on having a decent kiter because this is otherwise largely a tank and spank as well.
What your kiter needs to do is grab these three Shadows and keep them from agro'ing onto the Tank because they will each channel a stacking 24% healing debuff on the person in range who gains agro on them. The good, or bad news depending on how you want to look at it, is that these mobs are "Twitchy", meaning their agro table is set by not by traditional threat build mechanics but, instead, upon the last person who's hit them with a damaging attack. This makes it very easy to get their attention but you can also lose it very quickly if anyone else, particularly the tank, does anything at all to hurt them. So, it's a good idea to keep them at a distance while kiting. Complicating this a bit, Obsidius will "swap places" roughly twice a fight with one of his adds so you'll need to be prepared to pull your two remaining adds out again and pick up the add now standing where the boss was. Note: Searing Totem is bad for picking up this add and sending it running off towards the Shaman that owns it. It's a good idea to recall on the switches. Also, Tanks should be aware of the "Twitchy" mechanic and avoid breaking out AoE abilities during the switch; especially remember your glyph situation WRT things like Maul...
While your kiter is dealing with the Shadows, your Tank need merely hold the boss in place while everyone else burns him down. Obsidius will periodically Thunderclap and cast a DoT called Twilight Corruption on random party members which should be dispelled.
As long as your Kiter keeps the adds off the Tank, you'll be done in short order and moving on...