Hydross the Unstable, labeled ‘Duke of Currents,’ is the first boss of SSC(Serpent Shrine Cavern) and appears to be a high profile water elemental imprisoned by Vashj for some nefarious purpose, but none of this matters whatsoever as he drops epics.

Abilities

Hydross possesses a few similar abilities that simply change element depending on his exact position, that is, whenever he is within the line of the beams/at his starting point he will be in the water form, when moved from this position a short ways he enters poison form.

Water Form

Immune to Frost damage

Melee – 5,500 frost damage that is both unblockable/ignores armor as well as being unable to crush.
Mark of Hydross – Increases frost damage taken by 10, 25, 50, 100, 250, and finally 500 percent, 15 second cooldown and a 30 second duration, refreshing with each stack, affects the entire raid.
Water Tomb – Targeted 8 yard range AoE cast every 7 seconds, both stuns for 4 seconds and deals 4500 frost damage over 5 seconds.

Poison Form

Immune to Nature damage

Melee – 5,000 nature damage, unblockable/ignores armor as well as being unable to crush.
Mark of Corruption – Increases nature damage taken by 10, 25, 50, 100, 250, and finally 500 percent, 15 second cooldown and a 30 second duration, refreshing with each stack, affects the entire raid.
Vile Sludge – Targeted debuff, deals 4,000 nature damage over 24 seconds as well as reducing healing and damage done by 50%,
15 second cooldown.

Phase 1

Beginning the fight you’ll want to avoid triggering a premature transformation, having your designated water form tank(equipped with hopefully quite good resistance gear) either engage the boss using an invisibility potion or a speed buff/intercept of some sort to prevent him from going poison early on.

The fight itself is quite simple, due to the stacking debuff on your tank you will inevitably be required to retreat otherwise face being one-shotted.  This would generally be at the 100% damage increase mark so a minute into the fight/4 stacks of the debuff.

At this point your tank draws back, pulling hydross from his initial positioning and thus triggering the phase shift into poison form, a transformation accompanied by the summoning of 4 elementals.

Phase 2

Each elemental has around 50,000 HP and deals around 6,000 damage, frost or nature dependent upon the boss himself/his current form, when pulling him back into the beams and therefore water form they’ll be water/frost immune, when pulling him out posion/nature immune.

Have DPS go easy during each transition, focusing on dropping the summoned elementals quickly before returning to the boss and beating him down.  The main thing is that you need enough DPS to drop the boss before the 10 minute enrage marker.