I don't think hydra's fire damage can be absorbed, at least it didn't seem to when I tested it a few days ago. 2 dwarf stars 81 resist guardian angel rising sun and hellmouth.
4500 life, I was dead within 7 seconds.
Yes, hydra's fire damage is just like any other fire damage. I'm surprised you died so quickly though. Did you die previously in the game? Due the Anya quest bug, you lose 30 res (although you won't see it in the character screen) once you die in a game.
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