Before engaging, pop a hope token and use Dwarf-endurance. A huge factor in this fight is being able to conserve power. Use all blue on Fellowship Manoeuvres if you can.
The fight starts off and you are attacking Mordirith. He has a nasty knock around move that will launch you approximately 2/3 of the length of the platform you are fighting on. Make sure your back isn't facing the fire and you have good distance behind you. He can reset if he goes beyond the second row of pillars, so it may help to keep your backs to the side wall of his chamber.
At 75% of his health, he will spawn a wave of Ghostly Arms. These are generaly weak and you shouldn't have any trouble fighting them. If you need to, use Crowd Control (CC) on the last one to combat rez or rest to full. After the last one is dead, a wave of weak Mordirith clones will spawn. They all have very low morale so just use AoE to reveal the real Mordirith. At about 50% health, he spawns a wave of four Terrible Wood-trolls. These have nasty AoE so keep support classes and hunters back. These also have the knock around abilities so make sure you don't get thrown into the fire. CC the last one if needed to rest and rez.
Another wave of weak Mordirith clones will spawn, use AoE again and then continue the fight on Mordirith. At 25% health, he spawns two giants. These aren't too bad but they will knock you around as well. Fear and CC one while you focus on the other. Mez the last one for a while to recharge power/morale and reset timers, then finish him off.
After another wave of weak Mordiriths, you will eventually get him down to about 10% health and he will go green. A giant pillar of light will come from the floor inducing dread, and Mordirith will now be back to full, on his second form. He no longer will punt you around, so the chance of him resetting is much less likely. Just don't go near that big dread eye on the floor! He calls in other Mordiriths throughout the fight, but they are really weak and AoE will knock them out in one hit. Power reserves might be quite low at this point, but auto-attacks can still wear him down. A Lore-master or blue Fellowship Manoeuvre can be very helpful here.
At about 15% health, he will start spawning Fell Spirits. Either keep the spirits mez/CC and muscle your way through Mordirith throwing everything you have at him, or try to take down the spirits one by one and then focus on the boss. Eventually he will stop summoning them. During this time he will use his hope removal a lot. If the tank has his hope taken away someone should be ready to pop a token and the minstrel ready to cure. The tank will have severely reduced morale with all the dread, so abilities like Guardian's Pledge and Warrior's Heart, along with Endurance of Stone work wonders here. Minstrels can use Song of the Hopeful Heart to keep the dread down. If the tank does fall get him up ASAP. After Mordirith is defeated loot and hope for some decent treasure!
Short
Establish Hope order and buff up
Keep backs to the wall to avoid knock-backs
Rinse and repeat:
Defeat fakes
Burn him down to 75% / 50% / 25% morale
75% - Defeat crawling arms 50% - Defeat trolls (crowd control very important here!) 25% - Defeat giants (use crowd control again)
Defeat fakes
Defeat Mordirith's first form
Engage his second form Deal with stripped Hope Deal with adds
Defeat his second form and loot!
Strategy
Conserve power and having a token order is the key. Add hope as soon as it gets taken away. Also, make sure you don't get punted into the fire!