Quick guide for those reaching the final boss of this dungeon.

Before you pull, assign multiple "Signs" to each party member. For example, our tank used "Target to Attack" 1 and 2. Melee DPS used the Square and Circle signs. Ranged used "Target to Ignore" 1 and 2. Healer used "Target to Bind" 1 and 2.

Assign a color to each player. Red, Blue, Green, Yellow.

As you pull him, he will spawn doors around the arena. Each member is to match the two same-colored doors to each other at the very start. After this is done, the doors will close and hide their color away. DPS him as usual.

He will target 1 member of the party periodically and set a pulsing AOE node on top of them. Place these in the center of the arena.

He has a periodic AOE which will have safe spots both directly next to him as well as a fairly long distance away. Ranged DPS and healers need to be aware and stand roughly half of the arena's distance away to avoid taking damage.

He will periodically sleep the party, non-threatening but annoying.

At certain HP breakpoints, he will charge a very long skill which will hit your party VERY hard. To avoid all damage from this, open two matching doors while he charges it. The doors will remove your party from the fight temporarily, long enough to avoid damage, and place you back in.

Be sure you know who's opening doors in what order, and you should be golden.

Peace. Been fun figuring this out.

Credit goes to myself, Kallistra Noblus, Junko Akiyumi, and Avant Zero on Gilgamesh for the info.