Hydross the Unstable is the first boss of Serpentshrine Cavern in Coilfang Reservoir. He attacks solely with Frost-based damage in his Pure Form, and Nature-based damage in his Corrupted Form. Hydross also spawns 4 adds when he switches between each type; their damage matches the Form he changes to. In order to survive his magic-based attacks, the tanks require sufficient levels of resists.
The usual approach is to have one tank maxed on Frost Resist and one tank maxed on Nature resist; these two tanks will manage Hydross. There are also additional tanks with a mix of Frost and Nature resist to pick up the adds that Hydross spawns on each transitions.
Resistance Basics[]
365 is the resistance cap against a lvl 73 mob. At 365, it is considered to be at a ~70% average damage reduction (they will be resisting 75% of the damage 80% of the time - see Resistance for more information, but be aware that elemental melee damage does not behave exactly like spell damage). Hydross hits for ~8,000 unresisted, so this should make the average damage about 2400. The way resistance works, the amount of damage actually resisted will fluctuate from 75% resists to 25% resist; so healers remember that this is an average, and your tank will still take spikes.
Here are some suggestions on your resist sets.
You only need 365 resist against a level 73 mob anything more is WASTED. So you should only need the core plate set and a couple other items.
Frost resistance[]
Since the maximum effective resistance caps at 365 (anything over has no effect), you do not need every item listed, and can vary it slightly. Once you have all the items and you are down to enchants, if you choose only one enchant with +7, you will only obtain 364 resistance. If you go for the shield enchant, you can hit exactly 365 (including aura/totem), or you can go for two enchants and have effective 365. While it is untrue that being at 364 resistance puts you down to the 50% threshold, you should strive for 365, because going from 364 to 365 yields the greatest return on your time-to-live compared to going from 0 to 1. Note: This wasn't taking into account GotW from druids which most all raids will have available and conveys 33 resistance. With that taken into account you can forgo some of the expensive enchants.
A Shaman's Frost Resistance Totem can be used in place of the Paladin's Aura.
Nature resistance[]
One could always substitute a Savage Guard from Zul'Gurub for the legs augmentation, which will push the nature resist to 365 (366 actual, but effective is 365).
A Shaman's Nature Resistance Totem can be used in place of the Hunter's Aspect.
Druid Off Tank Gear[]
Despite the following table, most of a druid's non-crafted resistance gear will likely be BoE random enchantment gear from the AH - either of Frost Protection, or of Nature Protection. The rest should have resilience/defense rating, stamina, and agility. Resilience gear works very well for this fight, and the entire amount can be handled by [Timelapse Shard], [Alembic of Infernal Power], [Earthwarden], and [Formula: Enchant Chest - Major Resilience], leaving almost all slots and enchantments open for resistance.
Other buffs[]
Besides the group-based resist auras, there are additional options to raise your resistance. These are especially useful if your gear isn't complete yet or to be able to allow more gear options (increasing stamina, for example).
[Flask of Chromatic Wonder] - +35 to all resists, +18 to all stats- [Flask of Chromatic Resistance] - +25 to all resists
- [Broiled Bloodfin] - +8 to all resists
