All you have to do is look how each fight works to get a rough guess. Logs will simply verify the information to give a clearer picture.
Originally Posted by Lenox
Gormok's lethal damage comes from the bleed which armor doesn't mitigate. Fires should never be placed where the tank would get hit by it, and the rest of his damage is his melee. The stationary Jormungar will do the majority of his damage as magic (Fire or Poison), the mobile one will do most of his through Melee, with a portion coming from their sprays or poison clouds. Icehowl will rarely cast his breath on the tank, the vast majority of his damage is physical. At anyrate, the only lethal part (Ferocious Butt + Melee) is physical.
Done properly, the Jaraxxus tank will take virtually all physical damage. Looking at a few logs the past few weeks our tank took over 80% melee damage on Jaraxxus and the next highest source of damage came from Infernals charging to the melee and using their Fel Inferno (~8%)
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