Mechanics -

Soul Bomb: A deadly explosion that launches an AoE attack on a targeted aspect, dealing significant amounts of damage to everyone/everything around impact.

Death Dance: Loki enrages and begins attacking with extreme ferocity, dealing more damage & attacking much faster.

Passive Regeneration: As one of the later bosses, Loki has a base regen rate that makes it difficult for a team to proceed without adequate damage.

Teleport: Throughout the engagement, he'll teleport to random targets and change aggro.

Strategies - 

Tanks will need to keep the boss's aggro, and the ranged will have to spread out, lowering the number of allies struck by "Soul Bomb" if it is heaved toward one of the ranged DPS. When Loki teleports to a random target, it is the tank's job to use taunt and take aggro back. The most important skill to look for is Loki's "frenzy" that will more than likely kill most tanks. When he uses this, have your tanks begin to kite the boss and/or use stuns until the duration is expended. I recommend taking a Void/guile for their debuffs as well as a pure DPS class such as Greed, Ender, or Genesis to shave off that HP as quickly as possible. In regards to tanks, Brave is suitable due to being able to use Endure during frenzy, but the other tanks work, as well. If possible, bring a purity for the extra damage, but Omni & Light can be useful too.

Mechanics -

Rain of Fire: The boss will cast a rain of fire that lasts several volleys, dealing % damage to your health points so that even a tank will need to get out of the way to survive.

Mirror Image: The boss will summon a copy of himself that does a tenth of his damage, making the boss fight more chaotic and challenging to tank.

Passive Regeneration: As one of the later bosses, the fallen angel has a base regen rate that makes it difficult for a team to proceed without adequate damage.

Silence: The boss will randomly silence a target for about five seconds.

Fire Ball: A random ball of fire will spawn on the edge of the map, and target a player.  Run around avoiding the ball at all costs, and be careful not to lead it into allies!

Strategies -

The rain of fire is a simple mechanic. Have one of your dps stun the boss every time he casts his spell, or have a tank pull the angel out of the impact. When he summons a copy of himself, have one of your healers or DPS use Mark on the correct angel, rendering it easier for the dps to target the correct version. To discern which is the real angel, click on both and check their damage - the one with more is the real one. Composition doesn't really matter, just bring an aspect with a stun.

Mechanics -

Chomp: Small little worms will spawn from the middle of the map that rush toward the boss. When they reach the behemoth's mouth, he'll consume them and regen his HP. He also gains a damage boost.

Life Drain: Throughout the entirety of the dungeon, your life will slowly drain until you die. It's imperative to bring a healer to Devour!!.

Passive Regeneration: As one of the later bosses, the devourer has a base regen rate that makes it difficult for a team to proceed without adequate damage.

Waves: Periodically waves will spawn and spread across the area.  Avoid these, otherwise suffer immense damage.

Strategies -

The trickiest mechanic is, in part, his "consumption" ability. The key is to use a second tank to taunt the worms and keep them away from the boss. If a second tank is not amongst your group, then have your DPS swap targets & stun the worms until they die, preferable before they reach the boss. Be sure to bring a healer, as the constant life drain is a serious nuisance to DPS classes without a lot of HP or life steal.

Mechanics -

Focus: Focuses on a player for a set time and chases them down. Can be deadly depending on how much damage bonus the boss has.

Drink: The boss will try to go to the fountain and drink from it, increasing damage permanently

Desintegration cloud: A cloud of insects will chase a player down, if it touches the fountain they die.

Passive Regeneration: As one of the later bosses, the behemoth has a base regen rate that makes it difficult for a team to proceed without adequate damage.

Strategies -

This is a DPS run, if you allow the boss to drink too many times he will overpower the players, try to interrupt him or root him when he tries to go for a player or for a drink, positioning is also important here since the insect clouds can kill players if caught out of position.

Mechanics -

Infinite Slash: The boss will hurl a wind blade at a target that travels in a straight line, dealing a MASSIVE amount of damage that'll kill everyone aside from a fully-geared tank. To note, he will only use this slash on enemies auto-attacking him. It does not target spell casters.

Summoning: Throughout the boss encounter, the boss will summon monsters from the middle of the blackhole. Have your ranged DPS & healers stay on the outskirts to prevent dying from creeps.

Suplex: If your tanks or melee DPS spam "S" or "Stop" then the boss will grab them, drag them around the room, and suplex them as a finale.

Charge: Throughout the encounter, the boss will change targets and charge someone in the vicinity, dealing damage on impact and auto-attacking the victim until the tank reacquires aggro.

Gravity: Much like Infinite Slash, Gravity is a spell with a cooldown that the boss will frequently use. At one point or another, he'll pull someone to him, dealing damage and auto-attacking the target. The tanks will have to taunt to reacquire aggro.

Passive Regeneration: As one of the later bosses, the wolf boss has a base regen rate that makes it difficult for a team to proceed without adequate damage.

Strategies -

Whew, this boss seems a lot more complicated than the rest, huh? Well, it's because he is! The key to this fight is to have two tanks, one tanking the boss and facing him away from the group, and the other tank to grab aggro of the mobs that spawn in the middle. Every time he uses Infinite Slash, there'll be a 20 second period where you can damage the boss without worry. After which, his Infinite Slash will be off CD, and everyone needs to stop auto-attacking aside from the tank. Healers need to keep the tank at full HP to survive the slash, and once the slash is used, the DPS need to rinse and repeat the process. Have a healer or DPS countdown the CD for the group and paste "GET OUT!" every twenty seconds after the slash to make the boss a bit more palatable  for the team.

Mechanics -

Charge: The first boss will charge a target, dealing damage on impact and focusing the victim with his auto-attacks. Be sure to have a decent bit of armor before challenging this boss.

Bash: The final boss will randomly choose a target and chase after them, slamming them with their cudgel and stunning them for three seconds. It deals no damage as of 1.9.4

Passive Regeneration: As one of the later bosses,  the bosses inside Return have a base regen rate that makes it difficult for a team to proceed without adequate damage.

Strategies -

Tank the bosses toward the upper region of the zone, lowering the chances of spawned units from being aggroed and attacking the group. Have the range spread out a bit, and run away whenever you get charged. Tanks will need to try and pull aggro back as soon as possible. When the final bosses targets someone and walks towards them, just take the hit and let it happen - it deals no damage.

Mechanics -

Slash: The boss will walk toward a random target and slash them, usually killing anyone aside from a tank in one shot.

Encyrclement - The boss will summon towers that encircle a random player, killing them fast will help the player survive.

Regeneration Unit - the boss will summon a regenerative building, killing it fast ensures it doesn't get regeneration on his HP.

Passive Regeneration: As one of the later bosses,  this mechanical crab has a base regen rate that makes it difficult for a team to proceed without adequate damage.

Strategies -

Make sure your ranged DPS are paying attention and running away whenever the crab chases them, lowering the damage significantly. As for Melee DPS, have a guile in the team, or else you're going to die a lot. You can either gather together so you can aoe down the encyrclement units or be on a corner to better handle the mechanics. You also have to clear all of the mobs out before the boss becomes vulnerable.

Stats:

HP: 400,000,000

Damage: 30,000,000

Armor: 45,000

Mechanics -

Passive Regeneration: As one of the later bosses,  Burrowed Evil has a base regen rate that makes it difficult for a team to proceed without adequate damage.

Summoning: Burrowed Evil will summon monsters to assist it throughout the course of battle.

Reflecting shell: The boss will get a reflective shell every 25% Hp lost for a couple of secondas, attacking while this shell is up will deal heavy damage to the attacker.

Burrowed Evil: At some Hp intervals the boss will burrow and summon spikes beneath all players at random, avoiding them while avoiding the summoned mobs will keep you alive.

Berzerk: at low Hp the boss will berzerk greatly increasing damage and attack speed, kill it fast!

Strategies -

Have the tank face the boss away from the party, allowing Greed's and other DPS to deal as much damage as possible. When adds are summoned, have an off-tank grab them & let your AoE classes mow them down. Avoid the spikes when the boss burrows and don't attack when his reflective shell is active.

Stats:

HP: 800,000,000

Damage: 200,000,000

Armor: 47,000

Mechanics -

Flame Strike: Abezethibou calls down a rain of fire on enemies, dealing moderate damage.

Stomp: Abezethibou stomps the ground, dealing moderate damage on impact.

Calls for sister: Once the Abezethibou reaches low HP, he will call upon his sister for help, to which she will heal him to full hp.

Invulnerable:  The sister will be invulnerable when Abezethibou is above 30 million HP. Every time she comes out to heal him, there will be a short window in which she becomes vulnerable. Use this chance to deal as much damage as possible until she is dead.

Adds: The spawning monsters in this fight come from the road behind you, they will burn great amounts of mana from the units they attack.

Strategies -

Focus the brother until the sister becomes vulnerable, then use cooldowns and kill her as quickly as possible. Once she falls, there's no more healing for the doom-guard and he'll die soon after.

(Brother Abezethibou) Stats:

HP:  250,000,000

Damage: 90,000,000

Armor: 46,000

(Sister Abyzou) Stats:

HP:  150,000,000

Damage: 0

Armor: 46,000

First Boss : Slayer Warvil

Mechanics -
Disarm: Disarms a player, that player deals 90% reduced damage for a while.

Shackle: Shackles a player reducing attack and movement speed to a halt.

Calls for Prisioners: Once Warvil is reduced to 0 HP he will teleport out and come back to the fight with a new group of prisioners 5 times.

Rend:  Most prisioners will rend targets on hit for heavy blood loss damage.

(Slayer Warvil) Stats:

HP:  100,000,000

Damage: 940,000,000

Armor: 57,000

Second Boss: Magurlin

Mechanics -

Soul Chain: Holds a player with magical chains dealing heavy damage if not interrupted.

Blizzard: Deals damage on the target area.

Orb of lightning: Attacking one of the orbs can trigger a chain lightning that deals moderate damage to anyone that it chains to.

Extreme damage resistance:  Margulin has a very high damage resistance, but killing his orbs will drain 45% of his HP, timing both orb deaths correctly will remove 90% HP, delaying the kill of the second orb will only allow the boss to regenerate, since his regen is quite high.

(Margulin) Stats:

HP:  200,000,000

Damage: 540,000,000

Armor: 450,000

Third Boss: Corrupted Origin

Mechanics -
Corrupted Creation: Creates corrupted aspects to fight for the corrupted origin, each creation brings guardians to the fight.

Linked fate - Killing a corrupted aspect makes the Corrupted origin take heavy damage, continue killing them and eventually the origin will be left vulnerable, but be carefull every kill increases the damage done by the origin.

(Corrupted Origin) Stats:

HP:  299,000,000

Damage: 200,000,000

Armor: 500,000

Mechanics -

Doom Slash: Slashes on the direction of the indicated enemy dealing damage on the path of the created tornado.

Annihilation: Deal heavy damage to an enemy and extreme damage to any enemy near him.

Gehenna: Deals Massive damage to anyone in teh target area, this spell should be interrupted or at least avoided fast.

Raging Howl:  Decrease the damage of all nearby enemies by a great amount and gain armor.

True Ultimate Form:  Zealakana turns into his full form at 30% HP.

Ultimate Rage:  Zealakana deals more damage the lower HP he has, you better be fast when he is low HP or your tank will fall.

Strategies -

Interrupt Gehenna, dodge demon slashes and avoid being close together not to get 1 shot by annihilation, kill him fast at low HP or he will wipe the group.

(Stage 1) Stats:

HP:  300,000,000

Damage: 740,000,000

Armor: 100,000

(Stage 2) Stats:

HP:  900,000,000

Damage: 740,000,000

Armor: 135,000