This is a very elaborate 3 phase fight. For phases one and three you will alternate focusing down a huge pool of health and dealing with mechanics. For phase 2 you’ll have to alternate grouped movement and nuking down priority targets. Windwalker does very well here.
For phase one, your priority is to get bloods down after each breath.
Serenity is the better option for single target damage on Helya and few targets cleave on priority targets like Night Watch Mariner, tentacles and Grimelord. However, Storm, Earth, and Fire has a smoother learning curve, helps to deal with bloods after breaths if you struggle on it. If you feel very uncomfortable going Serenity on this fight, you might want to consider Storm, Earth, and Fire. Disregard the damage it deals to decaying minions phase two to judge its viability. This is not very relevant to killing the boss.
The best way to survive here is to have good knowledge of what’s going on and what will happen soon at all time. Being ready for Orb of Corruption fixates, being aware of Taint of the Sea debuffs, all things which will help you out immensely.
Diffuse Magic will act as a self dispel on a 2 minutes cooldown here. Be careful when using it as it will drop the Taint of the Sea void zone wherever you are. You don’t wanna deny positioning to your teammates.
If you’re not using Transcendence much, you can use it on top of the two upper platforms. This will allow you to bait Orb of Corruption there, avoid Fury of the Maw and jump back down or several other neat positioning tricks.
All the positioning tips are way more relevant here. Be very careful of where you are with Taint of the Sea and when you use Diffuse Magic. Dropping the debuff at the wrong place can mess up a lot of things.
Move ahead of schedule when the melee orbs are spawning, 4.5M ticks are quick to kill you.
The Bilewater Breath spawns orbs that are way deadlier than on lower difficulties. Luckily, we can easily jump from one target to another, gathering stacks of Spinning Crane Kick and cleaving 2 bloods and the boss with Fists of Fury or Strike of the Windlord. Always take advantage of the boss’s big hitbox.
It is stronger than ever here to have Transcendence ready to jump to the towers and bait Orb of Corruption out since these are the most important ability on mythic. Do so with care though and be ready to Diffuse Magic if things go wrong.
Decaying Minions on phase 2 have this neat particularity that people are not supposed to kill them until the raid is ready to deal with decay properly. This is a rare case where Windwalkers can stack up Spinning Crane Kick to very high amounts before AoE even start, giving us an edge. If you’re using Whirling Dragon Punch, don’t hesitate to pop Storm, Earth, and Fire a bit early and go Tiger Palm -> Blackout Kick to stack up chi and get 15-20 stacks. Even if Decaying Minions die instantly, using these 15 stacks will destroy Grasping Tentacles very fast.
This is the easiest fight to soak Touch of Karma damage as it is usually very easy to walk into water. You’ll take 500k damage per second on Heroic, closer to 1 million damage per second on Mythic. If using Cinidaria, the Symbiote, this will allow you to max out this scary legendary. I’d advise doing so intelligently.