When witch trying to kill the undead creature with the maximum hp, she is starting throwing the splash poison potions, which cannot inflict damage to undead creatures.
Witch should throw splash potions of regeneration or healing, but not the poison potions, which heals undead creatures instead of hurting them.
1. spawn a witch near the undead creature(I prefer zoglins, because they can easily aggro witches (it also works with skeletons(and other udead creature(if you able to aggro witch against the creature somehow))))
2. after witch will be aggresive towards the undead creature it will start throwing splah poison potions, which are harmless towards undeads
MC-94281 Witches can heal undead mobs