You have breached Doma Castle and rush in with your allies to reclaim it from the Empire.
How to Unlock
The Doma Castle 4-player dungeon is unlocked by the level 67 Main Scenario Quest The Die is Cast.
Garlean Fire is a series of circle attacks in a line. Move out of the way of where the arrows are pointing to avoid being in the path of the circle attacks.
Magitek Bits along the west side of the area will use line attacks across the room. Make sure not to stand in the line attack markers.
The next time the Magitek Bits attack, there will also be glowing Rearguard Mines along the north wall and south wall that float across the room. Make sure not to be in the path of any of the mines or you will be hit by Self-Detonate when it makes contact with you.
As the battle progresses, there will be more lines to dodge, and the Garlean Fire attack will consist of two side-by-side circles that move in the line of the arrow, so make sure to stay out of the path of both circles.
Arrive at the Training Grounds
As you progress to the next area, there will be gun turrets that periodically create line attack markers on the ground. Be prepared to move out of these as you proceed.
Magitek Hexadrone will use Circle of Death, a circle attack centered on the boss that will knock you back if it hits you.
A random player will be marked with a stack marker. Other players should make sure to stack together with the marked player to share the damage.
Magitek bits will make laser lines across the room. These do not do damage at first, but they will soon have line attack markers under them on the ground. If you step on these line attack markers, you will trigger Chain Mine, which will damage you and knock you back. Go to a safe area between the laser lines before they become line markers on the ground, and make sure not to step in the lines once they appear.
Shortly after the laser lines appear, a tower will appear in a random location in the area. At least one player needs to stand in this tower to reduce the damage from the Magitek Missiles explosion that occurs shortly after the tower appears. Make sure to move into the tower before the laser lines turn into line markers on the ground, to prevent getting hit by Chain Mine.
As the battle goes on, more towers will appear, so players need to make sure that at least one person is standing in each tower to reduce the damage from them.
Chainsaw is a continuous attack on the player with highest enmity. Grynewaht does not move after he begins this attack, so you can just move away from him to get out of the range of the attack and avoid further damage from it.
Delay-action charge will mark one or more players with a red circle marker that follows them. The marked players need to avoid overlapping other players with their circles.
Gunsaw is a continuous line attack. The targeted player should move away from other players to avoid putting them in the path of this attack.
Thermobaric Charge will mark a player with a circular marker and a "Prey" debuff. The marked player should go to a corner of the area away from the other players and wait for the "Prey" debuff to go away, at which point their circular marker will become a proximity attack marker on the ground. All players can then run to the far corner away from the proximity marker to minimize the damage from it.
Grynewaht will move to the center and glow with energy, which will make saw blades appear around the room. The saw blades will eventually have line attack markers showing where they will attack. Be prepared to move quickly out of the path of these line attack markers as soon as they appear.
There will also be Magitek bits that make laser lines appear across the room. As before, these do not do damage at first, but they will soon have line attack markers under them on the ground. If you step on these line attack markers, you will trigger Chain Mine, which will damage you and knock you back. Go to a safe area between the laser lines before they become line markers on the ground, and make sure not to step in the lines once they appear.
Completing the Quest
Defeating the boss will complete one of the quest objectives for the Main Scenario Quest Doma Castle. There will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.