Kadomi asked for some Thorim tips, so here goes.
Split your raid into two groups - six and four. The group of four is your tunnel team, and a warrior is probably better for this group. My Phase 1 experience is limited to the tunnel, so I will stick to that here. Take 2 dpsers and a healer with you. Before the first boss you get two groups of three mobs - each has two annoying melee and an annoying holy priest. The priest is always top priority for a kill because they heal, but conversely it's not really important to keep aggro on them because their damaging casts can (or at least could pre patch) be spell reflected, and they stop attacking and start healing once you get them to a certain level of health.
Charge/heroic throw the first priest and put some threat on him, then switch to pick up the adds. The priest will start off by trying to smite you, and will charge up a meaty heal once you get him to low health (it doesn't matter if someone overaggroes you here because he's healing - you can switch to the melee mobs). Someone (probably your dps) needs to interrupt his heals or to stun him. You will be busy targeting and picking up the other mobs. Watch out for the boss charging up his UP-like AOE, but frankly it is more problematic for your teammates than you since you should have the health to survive it (it is about 15k I think). The melee are annoying because they have a trip ability that is a short stun - they use this just before the AoE comes.
Once the priest is down in the second group of three, charge the boss, dragging the melee mobs with you. Face the boss away from the group, and nuke down the melee adds one after another before killing the boss.
The second tunnel then opens, with two groups of two mobs (priest/melee). More mobs will continually spawn from the top of the stairs. Again - put some threat down on the caster then let your dps finish him off while you make sure you pick up the melee mobs and any newly spawned adds. As before, when the second priest is dead you should aggro the boss to stop the spawning adds. Kill any surviving priests (there should be one perhaps, depending on your dps), then the melee adds, then the boss. I don't recall any particular ability of the boss that causes any trouble other than a lightbomb-style debuff that he places on one of you which means you need to avoid the rest of your group for a while.
Run into the final room (avoiding the circles on the floor as these are traps) and whack Thorim to make him jump down into the arena. The tunnel tank should pick him up, partly because you already have him targeted, and partly because there will be some arena mobs left to kill which will (hopefully!) be aggroed on the arena tank. Tank the boss with your back to the rear wall of the arena (just under the platform which you've jumped down from when exiting the tunnel. Both tanks should stand side by side - the tank who does not have aggro should watch carefully for the unbalancing strike debuff being applied to their partner, then taunt. I have a target-of-target frame in my UI, and it's easy to watch for the debuff icon on that.
The boss is just tank and spank from there. The key to surviving the tunnel is to not lose your dpsers. It can be done with just three of you, but it is not straightforward to do so because the mobs will not die fast enough.
Zeitgeist: The Dying Skyseer - Session 11
9 hours ago