This boss is divided into two phases. The strategy used when soloing is a bit confusing for many players as it involves resetting the encounter then restarting it.
Begin by killing off the mobs in the arena, which will subsequently activate the encounter. Open the door and run into the gauntlet using whatever speed buffs possible and pull the Iron Construct before running back into the arena. On the way back to the arena, the encounter should have reset due to noone present in the arena itself and Thorim will send a lightning orb through the gauntlet to kill off anyone remaining in it. Run back to the arena before the lightning orb catches up.
Once the construct is pulled to the arena, send your Shadowfiend or Mindbender on it and quickly run back into the gauntlet before the arena mobs respawn. The timing of the arena mobs respawn is important as the Mindbender must still be present to aggro them. Otherwise the player would be trapped within the gauntlet and it would be game over.
Once your Mindbender has aggro on all the arena mobs, they will begin to come after the player. Move through the gauntlet past the newly opened door (after the construct is dead) and make your way towards Thorim, while again killing off the arena mobs at the same time. Make sure to leave one arena mob alive until you are past the final gate to the gauntlet (opened after defeating the Rune Giant). Kill off the last arena mob to start the encounter again. Do not pull Thorim right away as this will bug out the encounter.
Immediately after Sif appears pull Thorim and jump back down into the arena. This must be done quickly as waiting even 3 seconds can result in death from the lightning orb again. Once Thorim is pulled, phase 2 begins.
During phase 2, Thorim is basically a tank and spank fight. However he will gain 15% increased damage as well as attack speed every 15 seconds. Therefore he must be nuked down fast. Don't bother wasting time to heal at the start aside from a few renews and shields. Save you defensive cooldowns such as Dispersion, Desperate Prayer and Vampiric Embrace until at least after the boss dips below 50%. If specced into Mindbender, he can be used twice during this phase. Flash heals can be used but its advised to keep its use to the minimum possible level. The fight ends when Thorim reaches around 1% health.