The fight against Thorim is split between two phases. The first one requires you to fight your way to Thorim by fighting multiple types of trash mobs in the Arena, the second one will let you face Thorim himself.
The first part of the fight is split in 3 mini-phases (Jormungar > Iron Dwarves > Hallway). Thorim will use a few abilities during the arena event, do not bother attacking him because he is protected by Sheath of Lightning.
You will first encounter a Jormungar Behemoth and a few NPCs from the opposite faction fighting in the Arena.
Once the NPCs and the Jormungar are dead, a lot of constructs (melee, casters, and healers) will jump into the arena from the stands and you will have to take care of them with only one part of your raid while the other part runs through the hallway on the left to reach Thorim.
As soon as you reach Thorim at the end of the hallway, he will jump in the Arena where the first part of your raid is and will engage them.
RECAP: 1.) Cast a couple of FOL every now and then. 2.) stay away from the 4 electric generators. 3.) profit 4.) Rinse and repeat.
Edit: since i went to the tunnel in last nights raid, and first time experianced the fights in it, its pretty much easy, but the healer and the tank needs to be on the toe for the heal/taunt the first encounter in the tunnel is a walkway, and in the end of it is a mini boss, his strategy is just like the boss in UP (Skadi), he starts casting with his right hand, you need to stay in the left side of the tunnel, when he casts with his left hand go to the right side of the tunnel, its that easy. after killing this Mini boss, another tunnel will be opened, and you can see 3 adds coming to you, the tank will pull them up and as many adds that comes while you are going up on the stairs, he must pull the boss so he will stop pulling adds, everyone there should stay away from each others, tank must have his back to the wall, and dps should use all their threat decreasing abilities as much as possible since the adds reset aggro every now and then, kill adds then kill the mini boss. mini boss does a knock back every 10 secs, it will stun you, then you'll explode, if you are near someone else, you'll kill him.
after killing that one, run to Thorim, WALK ON THE EDGE OF THE WAY, there is some big bad circles on the ground. after you pull Thorim, he will jump and the fight will start.