The main challenge on Thorim, is dividing the raid into two separate functional groups, and performing two simultaneous tasks. Too few people (or not the right people) in either group, and you A) get overwhelmed in the arena, or B) take too long in the hallway.
LOKI used a warrior tank in the hallway (myself) - something I hella recommend. Two charges, an interupt, two stuns, and a fear are amazingly useful (and at times, I felt like one less would have spelled disaster).
Here's how we did it...
10 people in the hallway. 1 tank, 2 Healers (Shaman & Paladin - insta-cast healers are better in the arena), and 7 ranged DPS (Any, but 1-2 Mages and 1-2 Warlocks is really helpful). 3 Tanks, 5-6 Healers, rest DPS in the arena.
The night we got a kill, we had just one mage. He took the left add by default in the firs two hallway groups. I would tab target the right melee add (you can do it through the wall, as you round the corner) - charge and slam him, then grab the caster. The casters always get burned first, then the non-CC'ed melee add (which can also be sheeped, if your rocking two mages), then the last one. I would break the CC as soon as I felt I had enough threat on the previous mob to ignore it, to keep things running fast.
This whole time, we had one ranged who's job it was to call out which side we should be on, to avoid his UP-style half-hallway filling blasts. The big danger to tanks, is when you've got up two mobs (caster/melee, or melee/melee), and one hits you with his knockdown/stun. You can't avoid attacks during this time (so your getting wailed on hard), and you can get stuned in the path of the incomming blast. Alone, the stun or blast isn't too bad (you can eat a blast if you have to get in its way to break the last CC, as long as you have little else on you).
The first mini boss is a snooze. Point him away (towards the door), have the raid stack up on/behind him, and blast away. Nothing to see here.
Then the door opens - two mobs at the bottom (caster/melee), and two more waiting up the stairs a bit. Tag the first two, aggro the second two. Have the melee sheeped/hexed if you can, and focus on killing the acolytes ASAP. As soon as the second two are aggro'ed, a hunter runs up and MD's the boss to you (which stops any additional adds from spawning). If you don't let them heal, the two acoloyes should be dead by the time the boss gets there. The melee can wail on you if you want - or you can fear them, stun them, disarm them, ect. Get the boss down, leave remaining melee adds in the dust, and go pull Thorim (avoiding the circles in his hallway). The melee adds will run all the way back down the hallway, and you'll have to kill them eventually, but there is work to be done in the arena.
(I've left out info on the Arena, since I wasn't in there. To the best of my knowledge, however, they just single target focused things down, with everyone standing in the very middle. Kill the whirlwinders first, then the warbringers, then the acolyes. Don't bother with AoE - just have an Enh Shaman purge spamming the current focus target to remove any shields / HoTs, and let the rest of them overheal themselves to their hearts content).
When you drop down into the arena, it will probably be ugly - Ttns of mobs, and a boss that just droped down. Take a moment and drop a fat shockwave stun on the pack of mobs, then get Thorim on you ASAP, as the other tanks are probably still busy.
Have one other tank that is going to trade off with you on Unbalancing Strikes (a designated one, for the sake of your healers). The other two, once adds are dead, should go hang out in a remote section of wall, to avoid chaining chain lightning into melee. If(when) a tank dies, have one of those set to move in and take their place.
When Thorim's self-buff (which increases damage and attack speed) gets to around 10+, things really get ugly. Your Indestructible Potions should definatly be active at this point, at a minimum. When you take Thorim, pop cooldowns preemptivly. Its going to get to the point of being a hardcore lottery as to if he kills you, which is why you have backup tanks. And following backup tanks, are evasion-ing rogues.
Its a dynamic fight from a tanks point. Warriors will be streched to use their whole range of abilities to move fast, prevent mobs from healing, and live through Thorim's damage. The rest of the raid has to bunch up in p1, spread out when Thorim comes down, kill him before he becomes too strong, and in general not die.
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