Well, maybe a bug, or maybe just a detail.
The 'legendary pet' buff says "reduces all damage taken by 50%." What it actually does is calculates 50% of the base damage of the incoming attack and subtracts that from the damage done. Most of the time it's the same thing, but not when there's a buff/debuff that multiplies damage. For example with Howl/Shattered Defenses which double damage, you'd think it'd work like this:
(normal damage)*2*0.5 = normal damage
but it actually works like this:
(normal damage)*2 - (0.5*normal damage) = 1.5 * normal damage
The upshot is that multiplicative buffs/debuffs are super effective against the Beasts of Fable: for example Black Claw + Hunting Party kills Lucky Yee in 3 turns easily.