Howling Abyss

First boss - Hydra
The boss will spawn eggs in the room from time to time, make sure to focus and kill them ASAP before they hatch, to avoid adds. Melee DPS should be wary as the boss has an AoE tail-attack that can make them take a lot of damage. The boss will cast ice patches on the floor beneath everyone, be sure to run away from them. After a few seconds standing in them they deal a hefty amount of damage and leave you frozen for a while, fully vulnerable. Finally, from time to time, the boss uses a charge, charging at a random member of the party. This can just be dodged, but if you make it charge into the barrels along the walls it will stun the boss so you can take it out more easily.

Second boss - Guard Captain
The second boss inflicts heavy bleeding with some attacks, pay attention to that. He will often start spinning around and jumping from people to people, there is nothing you can do about that. Healer(s) have to be ready to heal everyone when this happens. He can also start spinning and cast magical bullets away from him, you can easily dodge them if you're far from him. Melees should try to stay away from him when he spins, as there's no use taking unnecessary damage. He will spawn icy adds, DO NOT KILL ALL OF THEM. Keep a couple alive and kite them all around the arena, they are very slow. His most powerful move is when he starts spinning into a fire column, and then starts flying before causing a massive explosion. It deals A LOT of damage. You can counter it either with the level 50 Auramancy spell "Wings of Protection" or by getting close to an icy add. In a few seconds their aura will freeze you, and while frozen you are fully immune to the fire damage.

Third boss - Calleil the Wicked
Hardest boss of the dungeon. She only deals magical damage, so plate tanks will not be able to tank her as they will take way too much damage. A cloth (or leather) user has to tank her, ideally the person having the highest magical protection. There are 4 puddles in the arena, and from times to times, the boss will flood the arena with water. Waterspouts will emerge from the puddles, MAKE SURE to stand in them and get lifted off the water, or you will take intense damage. Note that while airborn you won't be able to use skills that have a cast time, so a healer with Spell Shield (which makes their main heal Antithesis instant cast) is extremely helpful. She will also place magic traps on the floor, which disable you (silence or stun) and leave you open to damage. The elementals she spawns are relatively harmless, so focus on the boss to take her down as fast as possible.

Fourth boss - Musperosa
The boss has an attack with a long cast time that will petrify everyone in the party. When he starts casting it make sure to grab one of the shields laying around against the walls, which when activated (you'll see a button prompt pop up to use it) protects allies in a small area and reflects the spell back to him, leaving him petrified instead. The shield is single-use, and will respawn at random locations along the wall. Don't use more than two shields at a time, and make sure to locate the next shield spawn as soon as possible to stay ready. Avoid the explosive shiny balls that get thrown, and watch out for his stomping attack, which deals very heavy damage to anyone near him.

When he gets enraged, make sure to stay away from him as his damage goes way up, and pick up a harpoon gun from the weapon rack (you can have one person carry it the whole fight, just know their movement speed will be reduced). Fire the harpoon at him to inflict stun and cancel rage mode. This boss summons no adds, so once the special mechanics are taken care of it's a fairly straightforward fight.