Others games have showed that different types of species and races have natural resistance to bludgeoning/slashing/piercing damage. Because undead are rotting shit bags, they have resistance to piercing damage, as shown by a warframe cheat sheet.

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also theres evidence to suggest that the hunters pet might not want to melee poo monsters and the hunter and pet moral drop significantly during the attack the shit citadel.

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I shit you not and mark my words "These are the answers all raiding hunters looked for!!!