Hydross the Unstable

Abilities


Summon elementals: Whenever Hydross switches form he summons 4 elementals matching his new form
Immune to taunt
Enrages after 10 minutes

Water form

Melee: 5.5k frost damage base, cannot be blocked, cannot crush
Immune to frost damage
Mark of Hydross: 15 sec cooldown, raid-wide debuff, stacks up to 6 times, increase frost damage by 10, 25, 50, 100, 250, and 500 percent. 30 sec duration, refreshes on each application
Water Tomb: single-target 8 yard AoE, cast every 7 seconds. Stuns all affected players for 4 seconds and deals 4500 frost damage over 5 seconds

Poison form

Melee: 5k nature damage base, cannot be blocked, cannot crush
Immune to nature damage
Mark of Corruption: 15 sec cooldown, raid-wide debuff, stacks up to 6 times, increase nature damage by 10, 25, 50, 100, 250, and 500 percent. 30 sec duration, refreshes on each application
Vile Sludge: Single target debuff, 15 sec cooldown. 4k nature damage over 24 sec, and reduces healing and damage done by 50%.
Elemental Adds
Health: ~50K
Damage: ~6K Frost/Nature base melee damage
Immune to nature/frost damage

Preparation

Raid Composition
1 main Tank with maxed frost resistance (see Hydross Resistance Gear)
1 main Tank with maxed nature resistance (see Hydross Resistance Gear)
2-4 offtanks with some assortment of nature and frost resistance gear
6-8 healers (preferably at least one shaman)
11-14 damage dealers (any mix of melee and ranged will do)
The frost tank has to take slightly more damage than the nature tank.
Positioning
The melee group needs to move with Hydross, on each phase change a few yards forward and backward. All other raid members should set up distributed, to reduce the effect of Water Tomb. It's also no problem to run through Hydross and set up on his back.

Starting the Fight

Prior to the pull a small group (one off-tank, a few damage dealers and a healer) need to pull and kill enough of the tainted water elementals on the "assembly line", so that none of them take part in the fight as adds. After the fight has started the tainted water elementals stop spawning.
Hydross starts in his water form. Whenever he leaves the area where he stands initially, he switches to the poison form. When he's moved back into the two beams, he switches back to the water form. Form switching takes place whenever he crosses a "magic boundary" which runs approximately between the two small flags in front of him. Every time he switches form, four adds of the same type as his new form are summoned. If pulled conventionally, Hydross would immediately switch to the poison form and summons adds.
Druid tanks can simply use Feral Charge and start the fight with a full rage bar. If druid is off-tank, main tank taunts aggro from him.
The safest method to avoid this is to use a Lesser Invisibility Potion. The frost tank has to wait until he drops out of combat (because he is in combat due to the assembly line adds being killed), use the potion, walk into melee range and start the fight from there.
Another possibility is to use a combination of Swiftness Potion and Intercept, to reach him fast enough before he crosses the boundary.
As a third safe method, a mage can use Invisibility, walk behind Hydross and Ice Block there. Hydross aggros on the ice blocked mage and the tank has 10 seconds to walk in and pick up Hydross. The drawback is that this uses the cooldowns on both Invisibility and Ice Block.

Switching Form

Due to the stacking debuff, the damage taken by the tank sooner or later reaches levels where he simply gets one-shotted. Usually 100% damage increase is the limit, in water form this means 22k crits are possible. Thus after 60 seconds (= four debuffs), it's time for the current tank to move backward and pull Hydross over the boundary.
This is a most critical moment, because immediately after a form switch Hydross resets his aggro list and summons four adds. There must be no DoTs on him and all DPS and healing must stop for a few seconds to give the tanks time to gain control of Hydross and the adds. If some raid member on the wrong side of the boundary draws aggro so that Hydross moves back over the boundary, he switches form again and summons four more adds (= wipe).
In water form he should be tanked at his initial position. There's a step running in front of his initial position, which forms an angle. Just down the step near that angle is a good tanking position for the poison form.

Elemental Adds

Quickly killing the four elemental adds is a key to success in this fight. Good tanking coupled with solid AoE damage is the fastest solution, particularly if the AoE also includes Hydross. The optimal approach to this would be two Paladin tanks, one in Nature and one in Frost resistance gear. Unfortunately raids will rarely ever boast this luxury, therefore most groups will have to find their own solutions to the problem. Druid tanks work well, some combination of AoE and emergency "point" tanking by warriors does the job too. Resist gear is also useful on the add tanks. The faster the adds are killed, the less damage they do (since their damage is increased by the raid-wide debuff too).

Notes

On the initial phase switch, Hydross should be at 80% or lower. If that's not the case, the raid's DPS is probably too low to kill him before he enrages.
A hunter's Frost Trap in Hydross's path while switching to poison form slows him for a few seconds and delays the spawn of the adds, which makes them easier to control.
Hunters are also very useful in the Poison tank's group for Aspect of the Wild to help mitigate some of the nature damage.
If he's at 15% or lower after a switch, it's possible to use all high damage abilities and trinkets, CC the adds and nuke Hydross to end the fight in that phase.
If the entire raid moves to the correct side of the platform, the danger of Hydross aggroing a player on the wrong side and switching forms again is nonexistent.
Because of changes in patch 3.0.2, a T4-T5 geared level 70 group can do this fight without switching phases until the debuff has stacked to much greater levels, and the tank doesn't need frost/nature resistance (though a hunter aspect and paladin aura still help). With our group we killed him after only switching him twice.