Well I guess that explains why that cramming 50 MDR on him did just about nothing. I guess the best options remains to be Guardian Angel with enough resists to hit 90%.Your Barbarian can negate any Hydra damage in Travincal with MDR 28 (164*0.17), since the player count damage increase only applies to attack or source damage, there's no critical hit chance and pets only apply 17% damage to players.
Hydras summoned by Geleb Flamefinger and his minions will still apply 135-164 fire damage per bolt to your mercenary, though.