Thorim

The fight against Thorim is split into two phases. The first one requires you to fight your way to Thorim by fighting multiple types of trash mobs in the Arena and Gauntlet, the second one will let you face Thorim himself.

The first part of the fight is split in three 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. You can not fail, just tank the mobs and AoE them down.

The raid will be split in half, one group (A) having one tank, one healer and two ranged DPS while the other one (B) has all the remaining players (Pref AoE dps).

Gauntlet and Arena events happen at the same time:

Gauntlet

After the mobs die, Thorim will stall babbling in confusion and Group A should pull the lever on the side of the arena and begin running through the gauntlet.

The gauntlet itself is pretty simple, the only thing you need to watch out for is the big mob in the end of the first hallway, he will constantly "explode" the sides of the rooms which deal hefty damage if you stand on the wrong side. You can see which side will explode by looking at the mob's hands, which have fire coming out of them before the explosion comes. Careful while killing the mob too, he has a damage-reflect buff.

The other big mob before the end isn't much of a threat, he places a buff on one of the players which immobilizes and deals damage after a few seconds.

In the last hallway hug the sides unless you want to get stunned for ~20 seconds. Overall, it's very easy and with four players, not a problem at all.

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.

Arena

While the gauntlet is being done, adds will spawn in the arena where the remaining players are. Basically, it's a big AoE zerg, where you target the sentinels first. Rogues are especially good in the arena, tricks/dismantle/aoe works wonders. Leave your AoE classes and those who can interrupt here. Interrupt heals, dispel HoT's and make sure you focus-burn the Vrykuls.

After Thorim is forced into the arena by the Gauntlet group, Phase 2 will begin.

Casters and Healers should spread out equally around the room, tanks should rotate the boss to avoid anyone being hit while with Unbalancing Strike, and everybody in the raid should get away from the lightning fields Thorim creates between himself and any point on the walls.

They will look like cones of small sparks, which will soon erupt into a Lightning Charge.

Lightning Charge - Charges a 60-degrees cone at random direction with lightning sparks. The cone will cause Lightning Charge after a few seconds, inflicting 17,344 to 20,156 Nature damage to players in it, and increasing Thorim's attack speed by 15% and Nature damage by 10%.

If you zoom out the minimap, you will see there are eight circles on the edges of the room. These are the circles from which the lightning comes out. It will look like two lines of lightning bolts coming from the wall straight to Thorim. A few seconds after that, lightning charge will strike on either side of where the bolts had come out. So move quarter of a circle away from the original line.

Melee DPS try to stay at maximum distance from the tank because chain lightning does a lot of damage. In hard mode, Thorim's chain lightning will start to one shot people after two jumps, so spreading around the room is very important.

Chain Lightning - Inflicts 3,700 to 4,300 Nature damage on its main target and increases its damage by 100% each time it jumps.

Unbalancing Strike - Inflicts 200% weapon damage on the main target and reduces its defense skill by 200 for 15 seconds.

Hard Mode - Sif

If you reach Thorim within 3 minutes, he will not be affected by Touch of Dominion and have 25% more damage and health. Additionally, he will be helped by his wife Sif. Sif cannot be attacked and will have the following spells:

Frostbolt Volley - Inflicts 7,200 to 8,800 Frost damage to all players and reduces their movement speed by 50% for 4 seconds.

Frost Nova - Inflicts 7,069 to 7,931 Frost damage to nearby enemies, immobilizing them for up to 6 seconds. This spell has 2.5 seconds cast time.

Blizzard - A rain of ice moves around the room, dealing 2,188 to 2,812 Frost damage every second, for 10 seconds. Targets caught in the Blizzard have their movement speed reduced by 65%. It cannot be dispelled. It moves around the room. If you get caught in it, the fastest way out is moving towards the centre of the room or walking through it against the movement.