Abyssal Commander Sivara
A common strategy for dealing with this encounter is to have the boss in the middle of the room and have clear halves of the room. One half is for Poison, the other for Frost.
When the encounter begins all players will be given a Frost Mark or Toxic Brand, you will have some time to position yourselves without consequence. After this initital grace period, if you collide with a player of the opposite type, or a mechanic of the opposite type you will trigger Unstable Mixture which will deal a small amount of damage to the entire raid for 3 seconds.
Frost Mark - This deals frost damage every second and slows movement speed by 5% per stack. You can reset your stacks by getting hit by a poison mechanic, or standing in Septic Ground however this will deal raid wide damage from Unstable Mixture.
The boss will swap which side is Frost + Toxic each cast. Position the boss in a way that it will always be on the "correct" side for your DPS/Healers. If your raid is positioned as noted in the image at the top of the page you will begin with the boss facing away from the enterance.
Crushing Reverberation - This will deal a large amount of physical damage on the active tank that is split between all players in a 5 yard radius. You will ideally want your melee that match your mark to soak this with you. After the hit lands all players will be knocked back, so ensure that you do not get knocked into Frozen/Septic Ground.
Frostshock Bolts - The boss will target non-tank members and throw an opposite spear (Frost Bolt / Toxic Bolt) at them. The non-active tank and melee can help to soak these if required. However if you soak one you will be pinned to the floor and need to be released by another player.
The type of spear and where its going is clearly indicated by a large arrow from the boss.
Inversion - When cast several players in the raid will have their marks swapped and be rooted and affected by Inversion Sickness. When the Invesion Sickness expires the marks of all players within 4 yards of them will be swapped.
Overflow - The boss will cast this on 2 non-tank players and they will get a 7 second debuff applied to them which will deal heavy Frost/Nature damage (depending on their mark) split between all players in 5 yards. When the Overflow triggers its damage the debuffed player will spawn a patch of Frozen/Septic Ground.
[Heroic] This will deal more damage requiring you to split the damage with other players (at least 3).
Tank Damage Profile
Place your Frozen/Septic Ground in a good location for when the debuff expires
Utilise defensives for Crushing Reverberation
Avoid touching the other tank or players with the other mark to reduce raid damage from Unstable Mixture
Position the boss so that Overwhelming Barrage hits the frost/toxic sides accordingly on the first cast. Each subsequent cast the boss will swap the sides around so rotate the boss 180 to ensure fewer people are accidentally hit by opposite bolts.
You can solo soak the Crushing Reverberation with the Azeroth's Undying Gift essence
You can solo soak the Crushing Reverberation with just Ironskin Brew
Vengeance Demon Hunter
Infernal Strike can be utilised to drop stacks in a safe location
Heroic Leap can be utilised to drop stacks in a safe location
The stacking of Frost Mark and Toxic Brand can be deflected with Spell Reflect
Tiger Dash can be utilised to drop stacks in a safe location
Blood Death Knight
utilise Anti-Magic Shell to drop your Septic Taint / Rimefrost stacks on top of an existing puddle to not gain extra stacks.