The encounter begins when the gang engages in combat with a group of 5 people from the opposing faction that is fighting a Jormungar Behemoth. Killing them is relatively easy, you just have to keep the worm with your back to the band and you have to kill the healer of the opposite faction while the rest of the soldiers are tanked until they are finished with.
Once you finish with them the band has to divide into 2. One group must stay in the Colosseum and kill the enemies of the Coliseum while the rest continue down the corridor to Thorim.
It's a good idea to organize your groups before breaking up by having a good mix of tanks, healers, and DPS in each group. It is best that most spellcasters go to the hallway and melee DPS and hunters stay in the hallway as they will not be affected by silence. Do not forget that at this time, Thorim will be throwing his hammer at the group in the Colosseum doing a lot of damage.
The group that remains in the arena must simply survive while the other group fights towards Thorim. The healers in this group must spread out so that only one healer can be silenced at a time.
The corridor group will need to go left towards the entrance on their way to Thorim. The door appears to be open only for a short period of time so the group must enter quickly and at the same time. As they proceed down the hall the group will come across the Rune Colossi who have a considerable amount of health and are small bosses that need to be wiped out quickly. Once you get to where Thorim is, the group will be paralyzed by magic bubbles, at which point Thorim will jump into the coliseum and start the fight against him. The group in the hallway must follow him and jump with him once they are released from paralysis.
Now the battle turns into a DPS race as he can kill a tank in one hit due to his Charge lightning if it is not killed in time. When a tank receives the Unbalancing blow, another tank should provoke it while the other has the negative effect. As the battle progresses the healers must heal more and more the tanks as the charges of Charge lightning.
However, if the group has managed to reach Thorim, they should have no problem finishing him off.