The Physical Damage Immunity on hostile creatures is supposed to be removed by applying Amplify Damage or Decrepify. While using The Reaper’s Toll on a mercenary, the Decrepify it casts as an on-hit proc, was NOT removing the physical damage immunity of some cursed targets.
Hi,
https://diablo-archive.fandom.com/wiki/Immune_Monsters_(Diablo_II)#Unbreakable_Immunities
I hope this helps.