On Mythic Difficulty, the Fel-Infused Siege Weapons spawn more frequently, and often two will spawn at the same time. When this occurs, one of the vehicles will emerge from the right side of the room to attack the right Hellfire Cannon, and the other will emerge from the left side of the room to attack the left Hellfire Cannon. See table for the Mythic Siege Weapon schedule, including which side of the room each vehicle spawns on.
The Mythic encounter also introduces a new Siege Weapon type, the Felfire Transporter, which delivers a regiment of a dozen Iron Dragoons.
As illustrated in the table, the section of the fight from 3 minutes in until 5 minutes in is marked by a reduction in frequency and number of Siege Weapons. These vehicles will concentrate in the center of the room, and also bring with them the two mini-bosses that players fought in the trash before the encounter.
The first Felfire Transporter carries Grand Corruptor U'rogg
Grand Corruptor U'rogg has the same abilities that he used in the trash, although Corruption Nova is now much more dangerous
In addition, Corruption Siphon will target two players with each cast, and it now heals U'rogg for approximately 3 times as much health as it dealt in damage
During this portion of the fight, U'rogg should take priority over everything else except the Felfire Crusher or an active Gorebound Terror
Healers should focus on healing off the Corruption Siphon healing-absorption debuff quickly, as multiple sources of raid damage occur during this portion of the fight
While U'rogg is still alive, players will also have to contend with a Felfire Crusher. This should take prioirty over U'rogg, as if it reaches a Hellfire Cannon, the raid will instantly wipe.
Grute's arrival via Felfire Demolisher
Grute is delivered by the Felfire Demolisher that spawns nearly 4 minutes into the encounter. He is jammed into the barrel of the Demolisher, and will be explosively launched towards the raid.
Unlike the trash version of Grute, this Grute does not use Cower!
Instead, he gains Shockwave, identical to the Shockwave ability that Siegemaster Mar'tak used at the very start of the encounter
This version of Grute also gains Cannonball - he marks a player with a large targeting reticule and will leap there a few seconds later
The reticule follows the player, and anyone within the reticule when Grute lands will take moderate damage
Grute has a large health pool, so Slam must be handled by tank-swapping
Players should focus on Grute after killing the Demolisher that delivered him, then clean up remaining adds
The Mythic strategy should be as follows:
Before beginning the fight, divide the raid into two equal groups and assign each group to defend either the right or left Hellfire Cannon
Use Bloodlust/ Heroism/ Time Warp and the DPS legendary ring at the start to push Siegemaster Mar'tak to 50% health before the first wave of Siege Weapons spawns
Once Mar'tak flees, focus down the Hulking Berserker, and, if there's time, kill Gorebound Felcasters one-at-a-time until the Siege Weapons arrive
Split into the two assigned groups to deal with the onslaught of Siege Weapons
The active Siege Weapon always takes priority, as they deal a lot of damage to the Hellfire Cannons while alive
Each side will have to focus down Hulking Berserkers before the tanks gain more than 5 stacks of Slam, and healers will need to use their external CDs on the tanks at 3+ stacks
Gorebound Felcasters should take second-top priority, and damage should be carefully controlled in order to prevent multiple, concurrent Terrors
When the Felfire Transporter spawns approximately 3 minutes into the fight, the raid should reconvene in the center of the encounter area
DPS cooldowns should be used again during this phase to burn down the Felfire Crusher and the two mini-bosses
Once Grute is dead, the raid should split back into its halves and continue to kill Siege Weapons and collect Felfire Munitions
The Reinforced Hellfire Door must take 67 shots, or 17 Felfire Munitions, to defeat the encounter
Players must kill at least the 10th wave of Siege Weapons in order to complete the encounter, making this an approximately 7-minute fight.
DPS legendary rings should be used nearly on cooldown to deal with the Siege Weapons, as the damage that the Weapons deal to the Hellfire Cannons is very high, and allowing them to live too long - or overlap at all - will likely result in a raid wipe.