For those who have difficulty with the rotation of the boss, here is the mechanics to help you understand and grasp the useful information. Thanks to the reddit user Syldris, we have learnt things from this guide.
Phase 1:
(boss does auto attack cleaves all the time. Should always be facing away from party members)
East Platform falls + Red Circle AoE's: Bait melee fire circles together at right side of South platform and then move away.
North Platform falls
West Platform falls + Red Circle AoE's: Bait again.
Phase 2:
Ice pad Spawns under the boss. You can't move through the boss.
Biting Halberd: needs to be directly behind the boss
Southern Cross: Stack at the butt to bait and then move out.
Biting Halberd/Ciclicle/Tail End: Randomly selected (skipped at 59%)
Phase 3:
Execrated Will = Should be tanked by a tank. Does high damage attack, which always crit, on its target and gain damage up buff every 10 sec. Use of Awareness helps mitigating damage especially when a single tank is getting both Wills in wave 2.
Execrated Thew = Small add. Should also be tanked by a tank. Nothing special.
Execrated Wile = doesn't need to be tanked. Should face away from it when Meracydian Fear is cast.
Execrated Wit = Doesn't need to be tanked. Should kill it before Meracydian Meteor casting is done.
Final Phase:
Tethers appear: 2 pairs of Infinite Fire and 2 pairs of Infinite Ice come out. Tethered pair should be close enough to each other to avoid DoT damage (about 10k dmg each tick). (similar to T11 tethers)
Wave Cannon: Randomly selects a non-MT player and does damage with Bleeding debuff. Everyone else should be away from its target, but make sure target's partner is in safe distance of tether.
Tyrfing: Attacks Main Tank 5 times with spear and does fire damage at the end. High Damages and cooldowns are needed as necessary.
Southern Cross: Stack under the boss and move out. Make sure tethered partner is going to the same direction or it's going to kill both players. Call out where you're going to!
Broken Seal: Spawns 4 fire meteors and 4 ice meteors. 4 players with Infinite Ice go into 4 Ice meteor circles while 4 Infinite Fire goes into 4 Fire meteor circles. (Match your color) Since players are tethered, tethered pair should always be close to each other. Call out! There are three patterns that randomly spawn
The Demon's Claw: Does High Damage and Stuns MT while putting Magical Vulnerability up. OT vokes the boss off MT. MT will also get knocked back. Make sure position yourself at the center so MT doesn't get knocked back into the fiery wall.
Wave Cannon: targets MT (not sure if it targets OT if he/she vokes during Demon's claw or MT still gets targeted by it) and shows huge AoE box that arrows indicate into the AoE. Seems like People stack in the box to soak the damage together. However, my group just killed MT and had OT voking the boss after. <basically repeating again from here>
New tethers appear: find a new partner
Wave Cannon: targets a random non-MT player. Away from its target.
Hope you can get useful information from above. Have fun with your game.